* [PATCH 6.1.y 4/4] mm/hugetlb: fix excessive IPI broadcasts when unsharing PMD tables using mmu_gather
2026-02-18 9:16 ` [PATCH 6.1.y 0/4] mm/hugetlb: fixes for PMD table sharing (incl. using mmu_gather) David Hildenbrand (Arm)
` (2 preceding siblings ...)
2026-02-18 9:16 ` [PATCH 6.1.y 3/4] mm/hugetlb: fix two comments related to huge_pmd_unshare() David Hildenbrand (Arm)
@ 2026-02-18 9:16 ` David Hildenbrand (Arm)
3 siblings, 0 replies; 5+ messages in thread
From: David Hildenbrand (Arm) @ 2026-02-18 9:16 UTC (permalink / raw)
To: stable
Cc: linux-mm, David Hildenbrand (Red Hat),
Laurence Oberman, Harry Yoo, Lorenzo Stoakes, Lance Yang,
Liu Shixin, Oscar Salvador, Rik van Riel, Andrew Morton
From: "David Hildenbrand (Red Hat)" <david@kernel.org>
As reported, ever since commit 1013af4f585f ("mm/hugetlb: fix
huge_pmd_unshare() vs GUP-fast race") we can end up in some situations
where we perform so many IPI broadcasts when unsharing hugetlb PMD page
tables that it severely regresses some workloads.
In particular, when we fork()+exit(), or when we munmap() a large
area backed by many shared PMD tables, we perform one IPI broadcast per
unshared PMD table.
There are two optimizations to be had:
(1) When we process (unshare) multiple such PMD tables, such as during
exit(), it is sufficient to send a single IPI broadcast (as long as
we respect locking rules) instead of one per PMD table.
Locking prevents that any of these PMD tables could get reused before
we drop the lock.
(2) When we are not the last sharer (> 2 users including us), there is
no need to send the IPI broadcast. The shared PMD tables cannot
become exclusive (fully unshared) before an IPI will be broadcasted
by the last sharer.
Concurrent GUP-fast could walk into a PMD table just before we
unshared it. It could then succeed in grabbing a page from the
shared page table even after munmap() etc succeeded (and supressed
an IPI). But there is not difference compared to GUP-fast just
sleeping for a while after grabbing the page and re-enabling IRQs.
Most importantly, GUP-fast will never walk into page tables that are
no-longer shared, because the last sharer will issue an IPI
broadcast.
(if ever required, checking whether the PUD changed in GUP-fast
after grabbing the page like we do in the PTE case could handle
this)
So let's rework PMD sharing TLB flushing + IPI sync to use the mmu_gather
infrastructure so we can implement these optimizations and demystify the
code at least a bit. Extend the mmu_gather infrastructure to be able to
deal with our special hugetlb PMD table sharing implementation.
To make initialization of the mmu_gather easier when working on a single
VMA (in particular, when dealing with hugetlb), provide
tlb_gather_mmu_vma().
We'll consolidate the handling for (full) unsharing of PMD tables in
tlb_unshare_pmd_ptdesc() and tlb_flush_unshared_tables(), and track
in "struct mmu_gather" whether we had (full) unsharing of PMD tables.
Because locking is very special (concurrent unsharing+reuse must be
prevented), we disallow deferring flushing to tlb_finish_mmu() and instead
require an explicit earlier call to tlb_flush_unshared_tables().
From hugetlb code, we call huge_pmd_unshare_flush() where we make sure
that the expected lock protecting us from concurrent unsharing+reuse is
still held.
Check with a VM_WARN_ON_ONCE() in tlb_finish_mmu() that
tlb_flush_unshared_tables() was properly called earlier.
Document it all properly.
Notes about tlb_remove_table_sync_one() interaction with unsharing:
There are two fairly tricky things:
(1) tlb_remove_table_sync_one() is a NOP on architectures without
CONFIG_MMU_GATHER_RCU_TABLE_FREE.
Here, the assumption is that the previous TLB flush would send an
IPI to all relevant CPUs. Careful: some architectures like x86 only
send IPIs to all relevant CPUs when tlb->freed_tables is set.
The relevant architectures should be selecting
MMU_GATHER_RCU_TABLE_FREE, but x86 might not do that in stable
kernels and it might have been problematic before this patch.
Also, the arch flushing behavior (independent of IPIs) is different
when tlb->freed_tables is set. Do we have to enlighten them to also
take care of tlb->unshared_tables? So far we didn't care, so
hopefully we are fine. Of course, we could be setting
tlb->freed_tables as well, but that might then unnecessarily flush
too much, because the semantics of tlb->freed_tables are a bit
fuzzy.
This patch changes nothing in this regard.
(2) tlb_remove_table_sync_one() is not a NOP on architectures with
CONFIG_MMU_GATHER_RCU_TABLE_FREE that actually don't need a sync.
Take x86 as an example: in the common case (!pv, !X86_FEATURE_INVLPGB)
we still issue IPIs during TLB flushes and don't actually need the
second tlb_remove_table_sync_one().
This optimized can be implemented on top of this, by checking e.g., in
tlb_remove_table_sync_one() whether we really need IPIs. But as
described in (1), it really must honor tlb->freed_tables then to
send IPIs to all relevant CPUs.
Notes on TLB flushing changes:
(1) Flushing for non-shared PMD tables
We're converting from flush_hugetlb_tlb_range() to
tlb_remove_huge_tlb_entry(). Given that we properly initialize the
MMU gather in tlb_gather_mmu_vma() to be hugetlb aware, similar to
__unmap_hugepage_range(), that should be fine.
(2) Flushing for shared PMD tables
We're converting from various things (flush_hugetlb_tlb_range(),
tlb_flush_pmd_range(), flush_tlb_range()) to tlb_flush_pmd_range().
tlb_flush_pmd_range() achieves the same that
tlb_remove_huge_tlb_entry() would achieve in these scenarios.
Note that tlb_remove_huge_tlb_entry() also calls
__tlb_remove_tlb_entry(), however that is only implemented on
powerpc, which does not support PMD table sharing.
Similar to (1), tlb_gather_mmu_vma() should make sure that TLB
flushing keeps on working as expected.
Further, note that the ptdesc_pmd_pts_dec() in huge_pmd_share() is not a
concern, as we are holding the i_mmap_lock the whole time, preventing
concurrent unsharing. That ptdesc_pmd_pts_dec() usage will be removed
separately as a cleanup later.
There are plenty more cleanups to be had, but they have to wait until
this is fixed.
[david@kernel.org: fix kerneldoc]
Link: https://lkml.kernel.org/r/f223dd74-331c-412d-93fc-69e360a5006c@kernel.org
Link: https://lkml.kernel.org/r/20251223214037.580860-5-david@kernel.org
Fixes: 1013af4f585f ("mm/hugetlb: fix huge_pmd_unshare() vs GUP-fast race")
Signed-off-by: David Hildenbrand (Red Hat) <david@kernel.org>
Reported-by: Uschakow, Stanislav" <suschako@amazon.de>
Closes: https://lore.kernel.org/all/4d3878531c76479d9f8ca9789dc6485d@amazon.de/
Tested-by: Laurence Oberman <loberman@redhat.com>
Acked-by: Harry Yoo <harry.yoo@oracle.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liu Shixin <liushixin2@huawei.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Rik van Riel <riel@surriel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 8ce720d5bd91e9dc16db3604aa4b1bf76770a9a1)
[ David: We don't have ptdesc and the wrappers, so work directly on
page->pt_share_count and pass "struct page" instead of "struct ptdesc".
CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING is still called
CONFIG_ARCH_WANT_HUGE_PMD_SHARE and is set even without
CONFIG_HUGETLB_PAGE. move_hugetlb_page_tables() still uses
flush_tlb_range() instead of flush_hugetlb_tlb_range(). Some smaller
contextual stuff. ]
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
include/asm-generic/tlb.h | 77 +++++++++++++++++++++++-
include/linux/hugetlb.h | 15 +++--
include/linux/mm_types.h | 1 +
mm/hugetlb.c | 122 ++++++++++++++++++++++----------------
mm/mmu_gather.c | 33 +++++++++++
mm/rmap.c | 25 +++++---
6 files changed, 207 insertions(+), 66 deletions(-)
diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index cab7cfebf40b..63d2c14ff277 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -46,7 +46,8 @@
*
* The mmu_gather API consists of:
*
- * - tlb_gather_mmu() / tlb_gather_mmu_fullmm() / tlb_finish_mmu()
+ * - tlb_gather_mmu() / tlb_gather_mmu_fullmm() / tlb_gather_mmu_vma() /
+ * tlb_finish_mmu()
*
* start and finish a mmu_gather
*
@@ -309,6 +310,20 @@ struct mmu_gather {
unsigned int vma_huge : 1;
unsigned int vma_pfn : 1;
+ /*
+ * Did we unshare (unmap) any shared page tables? For now only
+ * used for hugetlb PMD table sharing.
+ */
+ unsigned int unshared_tables : 1;
+
+ /*
+ * Did we unshare any page tables such that they are now exclusive
+ * and could get reused+modified by the new owner? When setting this
+ * flag, "unshared_tables" will be set as well. For now only used
+ * for hugetlb PMD table sharing.
+ */
+ unsigned int fully_unshared_tables : 1;
+
unsigned int batch_count;
#ifndef CONFIG_MMU_GATHER_NO_GATHER
@@ -345,6 +360,7 @@ static inline void __tlb_reset_range(struct mmu_gather *tlb)
tlb->cleared_pmds = 0;
tlb->cleared_puds = 0;
tlb->cleared_p4ds = 0;
+ tlb->unshared_tables = 0;
/*
* Do not reset mmu_gather::vma_* fields here, we do not
* call into tlb_start_vma() again to set them if there is an
@@ -424,7 +440,7 @@ static inline void tlb_flush_mmu_tlbonly(struct mmu_gather *tlb)
* these bits.
*/
if (!(tlb->freed_tables || tlb->cleared_ptes || tlb->cleared_pmds ||
- tlb->cleared_puds || tlb->cleared_p4ds))
+ tlb->cleared_puds || tlb->cleared_p4ds || tlb->unshared_tables))
return;
tlb_flush(tlb);
@@ -680,6 +696,63 @@ static inline bool huge_pmd_needs_flush(pmd_t oldpmd, pmd_t newpmd)
}
#endif
+#if defined(CONFIG_ARCH_WANT_HUGE_PMD_SHARE) && defined(CONFIG_HUGETLB_PAGE)
+static inline void tlb_unshare_pmd_ptdesc(struct mmu_gather *tlb, struct page *pt,
+ unsigned long addr)
+{
+ /*
+ * The caller must make sure that concurrent unsharing + exclusive
+ * reuse is impossible until tlb_flush_unshared_tables() was called.
+ */
+ VM_WARN_ON_ONCE(!atomic_read(&pt->pt_share_count));
+ atomic_dec(&pt->pt_share_count);
+
+ /* Clearing a PUD pointing at a PMD table with PMD leaves. */
+ tlb_flush_pmd_range(tlb, addr & PUD_MASK, PUD_SIZE);
+
+ /*
+ * If the page table is now exclusively owned, we fully unshared
+ * a page table.
+ */
+ if (!atomic_read(&pt->pt_share_count))
+ tlb->fully_unshared_tables = true;
+ tlb->unshared_tables = true;
+}
+
+static inline void tlb_flush_unshared_tables(struct mmu_gather *tlb)
+{
+ /*
+ * As soon as the caller drops locks to allow for reuse of
+ * previously-shared tables, these tables could get modified and
+ * even reused outside of hugetlb context, so we have to make sure that
+ * any page table walkers (incl. TLB, GUP-fast) are aware of that
+ * change.
+ *
+ * Even if we are not fully unsharing a PMD table, we must
+ * flush the TLB for the unsharer now.
+ */
+ if (tlb->unshared_tables)
+ tlb_flush_mmu_tlbonly(tlb);
+
+ /*
+ * Similarly, we must make sure that concurrent GUP-fast will not
+ * walk previously-shared page tables that are getting modified+reused
+ * elsewhere. So broadcast an IPI to wait for any concurrent GUP-fast.
+ *
+ * We only perform this when we are the last sharer of a page table,
+ * as the IPI will reach all CPUs: any GUP-fast.
+ *
+ * Note that on configs where tlb_remove_table_sync_one() is a NOP,
+ * the expectation is that the tlb_flush_mmu_tlbonly() would have issued
+ * required IPIs already for us.
+ */
+ if (tlb->fully_unshared_tables) {
+ tlb_remove_table_sync_one();
+ tlb->fully_unshared_tables = false;
+ }
+}
+#endif
+
#endif /* CONFIG_MMU */
#endif /* _ASM_GENERIC__TLB_H */
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 2266c34bcbc0..e9549a3b9073 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -207,8 +207,9 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
pte_t *huge_pte_offset(struct mm_struct *mm,
unsigned long addr, unsigned long sz);
unsigned long hugetlb_mask_last_page(struct hstate *h);
-int huge_pmd_unshare(struct mm_struct *mm, struct vm_area_struct *vma,
- unsigned long addr, pte_t *ptep);
+int huge_pmd_unshare(struct mmu_gather *tlb, struct vm_area_struct *vma,
+ unsigned long addr, pte_t *ptep);
+void huge_pmd_unshare_flush(struct mmu_gather *tlb, struct vm_area_struct *vma);
void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
unsigned long *start, unsigned long *end);
struct page *follow_huge_addr(struct mm_struct *mm, unsigned long address,
@@ -262,13 +263,17 @@ static inline struct address_space *hugetlb_page_mapping_lock_write(
return NULL;
}
-static inline int huge_pmd_unshare(struct mm_struct *mm,
- struct vm_area_struct *vma,
- unsigned long addr, pte_t *ptep)
+static inline int huge_pmd_unshare(struct mmu_gather *tlb,
+ struct vm_area_struct *vma, unsigned long addr, pte_t *ptep)
{
return 0;
}
+static inline void huge_pmd_unshare_flush(struct mmu_gather *tlb,
+ struct vm_area_struct *vma)
+{
+}
+
static inline void adjust_range_if_pmd_sharing_possible(
struct vm_area_struct *vma,
unsigned long *start, unsigned long *end)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 9b64610eddcc..d1a370add9bc 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -845,6 +845,7 @@ static inline void vma_iter_init(struct vma_iterator *vmi,
struct mmu_gather;
extern void tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm);
extern void tlb_gather_mmu_fullmm(struct mmu_gather *tlb, struct mm_struct *mm);
+void tlb_gather_mmu_vma(struct mmu_gather *tlb, struct vm_area_struct *vma);
extern void tlb_finish_mmu(struct mmu_gather *tlb);
struct vm_fault;
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 86218b9e647b..161f95473c2a 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5255,7 +5255,7 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
unsigned long last_addr_mask;
pte_t *src_pte, *dst_pte;
struct mmu_notifier_range range;
- bool shared_pmd = false;
+ struct mmu_gather tlb;
mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, mm, old_addr,
old_end);
@@ -5265,6 +5265,7 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
* range.
*/
flush_cache_range(vma, range.start, range.end);
+ tlb_gather_mmu_vma(&tlb, vma);
mmu_notifier_invalidate_range_start(&range);
last_addr_mask = hugetlb_mask_last_page(h);
@@ -5281,8 +5282,7 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
if (huge_pte_none(huge_ptep_get(src_pte)))
continue;
- if (huge_pmd_unshare(mm, vma, old_addr, src_pte)) {
- shared_pmd = true;
+ if (huge_pmd_unshare(&tlb, vma, old_addr, src_pte)) {
old_addr |= last_addr_mask;
new_addr |= last_addr_mask;
continue;
@@ -5293,15 +5293,16 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
break;
move_huge_pte(vma, old_addr, new_addr, src_pte, dst_pte);
+ tlb_remove_huge_tlb_entry(h, &tlb, src_pte, old_addr);
}
- if (shared_pmd)
- flush_tlb_range(vma, range.start, range.end);
- else
- flush_tlb_range(vma, old_end - len, old_end);
+ tlb_flush_mmu_tlbonly(&tlb);
+ huge_pmd_unshare_flush(&tlb, vma);
+
mmu_notifier_invalidate_range_end(&range);
i_mmap_unlock_write(mapping);
hugetlb_vma_unlock_write(vma);
+ tlb_finish_mmu(&tlb);
return len + old_addr - old_end;
}
@@ -5320,7 +5321,6 @@ static void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct
unsigned long sz = huge_page_size(h);
struct mmu_notifier_range range;
unsigned long last_addr_mask;
- bool force_flush = false;
WARN_ON(!is_vm_hugetlb_page(vma));
BUG_ON(start & ~huge_page_mask(h));
@@ -5350,10 +5350,8 @@ static void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct
}
ptl = huge_pte_lock(h, mm, ptep);
- if (huge_pmd_unshare(mm, vma, address, ptep)) {
+ if (huge_pmd_unshare(tlb, vma, address, ptep)) {
spin_unlock(ptl);
- tlb_flush_pmd_range(tlb, address & PUD_MASK, PUD_SIZE);
- force_flush = true;
address |= last_addr_mask;
continue;
}
@@ -5431,14 +5429,7 @@ static void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct
mmu_notifier_invalidate_range_end(&range);
tlb_end_vma(tlb, vma);
- /*
- * There is nothing protecting a previously-shared page table that we
- * unshared through huge_pmd_unshare() from getting freed after we
- * release i_mmap_rwsem, so flush the TLB now. If huge_pmd_unshare()
- * succeeded, flush the range corresponding to the pud.
- */
- if (force_flush)
- tlb_flush_mmu_tlbonly(tlb);
+ huge_pmd_unshare_flush(tlb, vma);
}
void __unmap_hugepage_range_final(struct mmu_gather *tlb,
@@ -6666,11 +6657,11 @@ long hugetlb_change_protection(struct vm_area_struct *vma,
pte_t pte;
struct hstate *h = hstate_vma(vma);
long pages = 0, psize = huge_page_size(h);
- bool shared_pmd = false;
struct mmu_notifier_range range;
unsigned long last_addr_mask;
bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;
+ struct mmu_gather tlb;
/*
* In the case of shared PMDs, the area to flush could be beyond
@@ -6683,6 +6674,7 @@ long hugetlb_change_protection(struct vm_area_struct *vma,
BUG_ON(address >= end);
flush_cache_range(vma, range.start, range.end);
+ tlb_gather_mmu_vma(&tlb, vma);
mmu_notifier_invalidate_range_start(&range);
hugetlb_vma_lock_write(vma);
@@ -6705,7 +6697,7 @@ long hugetlb_change_protection(struct vm_area_struct *vma,
break;
}
ptl = huge_pte_lock(h, mm, ptep);
- if (huge_pmd_unshare(mm, vma, address, ptep)) {
+ if (huge_pmd_unshare(&tlb, vma, address, ptep)) {
/*
* When uffd-wp is enabled on the vma, unshare
* shouldn't happen at all. Warn about it if it
@@ -6714,7 +6706,6 @@ long hugetlb_change_protection(struct vm_area_struct *vma,
WARN_ON_ONCE(uffd_wp || uffd_wp_resolve);
pages++;
spin_unlock(ptl);
- shared_pmd = true;
address |= last_addr_mask;
continue;
}
@@ -6762,6 +6753,7 @@ long hugetlb_change_protection(struct vm_area_struct *vma,
pte = huge_pte_clear_uffd_wp(pte);
huge_ptep_modify_prot_commit(vma, address, ptep, old_pte, pte);
pages++;
+ tlb_remove_huge_tlb_entry(h, &tlb, ptep, address);
} else {
/* None pte */
if (unlikely(uffd_wp))
@@ -6773,16 +6765,9 @@ long hugetlb_change_protection(struct vm_area_struct *vma,
cond_resched();
}
- /*
- * There is nothing protecting a previously-shared page table that we
- * unshared through huge_pmd_unshare() from getting freed after we
- * release i_mmap_rwsem, so flush the TLB now. If huge_pmd_unshare()
- * succeeded, flush the range corresponding to the pud.
- */
- if (shared_pmd)
- flush_hugetlb_tlb_range(vma, range.start, range.end);
- else
- flush_hugetlb_tlb_range(vma, start, end);
+
+ tlb_flush_mmu_tlbonly(&tlb);
+ huge_pmd_unshare_flush(&tlb, vma);
/*
* No need to call mmu_notifier_invalidate_range() we are downgrading
* page table protection not changing it to point to a new page.
@@ -6792,6 +6777,7 @@ long hugetlb_change_protection(struct vm_area_struct *vma,
i_mmap_unlock_write(vma->vm_file->f_mapping);
hugetlb_vma_unlock_write(vma);
mmu_notifier_invalidate_range_end(&range);
+ tlb_finish_mmu(&tlb);
return pages << h->order;
}
@@ -7130,18 +7116,27 @@ pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma,
return pte;
}
-/*
- * unmap huge page backed by shared pte.
+/**
+ * huge_pmd_unshare - Unmap a pmd table if it is shared by multiple users
+ * @tlb: the current mmu_gather.
+ * @vma: the vma covering the pmd table.
+ * @addr: the address we are trying to unshare.
+ * @ptep: pointer into the (pmd) page table.
+ *
+ * Called with the page table lock held, the i_mmap_rwsem held in write mode
+ * and the hugetlb vma lock held in write mode.
*
- * Called with page table lock held.
+ * Note: The caller must call huge_pmd_unshare_flush() before dropping the
+ * i_mmap_rwsem.
*
- * returns: 1 successfully unmapped a shared pte page
- * 0 the underlying pte page is not shared, or it is the last user
+ * Returns: 1 if it was a shared PMD table and it got unmapped, or 0 if it
+ * was not a shared PMD table.
*/
-int huge_pmd_unshare(struct mm_struct *mm, struct vm_area_struct *vma,
- unsigned long addr, pte_t *ptep)
+int huge_pmd_unshare(struct mmu_gather *tlb, struct vm_area_struct *vma,
+ unsigned long addr, pte_t *ptep)
{
unsigned long sz = huge_page_size(hstate_vma(vma));
+ struct mm_struct *mm = vma->vm_mm;
pgd_t *pgd = pgd_offset(mm, addr);
p4d_t *p4d = p4d_offset(pgd, addr);
pud_t *pud = pud_offset(p4d, addr);
@@ -7154,18 +7149,35 @@ int huge_pmd_unshare(struct mm_struct *mm, struct vm_area_struct *vma,
return 0;
pud_clear(pud);
- /*
- * Once our caller drops the rmap lock, some other process might be
- * using this page table as a normal, non-hugetlb page table.
- * Wait for pending gup_fast() in other threads to finish before letting
- * that happen.
- */
- tlb_remove_table_sync_one();
- atomic_dec(&virt_to_page(ptep)->pt_share_count);
+ tlb_unshare_pmd_ptdesc(tlb, virt_to_page(ptep), addr);
+
mm_dec_nr_pmds(mm);
return 1;
}
+/*
+ * huge_pmd_unshare_flush - Complete a sequence of huge_pmd_unshare() calls
+ * @tlb: the current mmu_gather.
+ * @vma: the vma covering the pmd table.
+ *
+ * Perform necessary TLB flushes or IPI broadcasts to synchronize PMD table
+ * unsharing with concurrent page table walkers.
+ *
+ * This function must be called after a sequence of huge_pmd_unshare()
+ * calls while still holding the i_mmap_rwsem.
+ */
+void huge_pmd_unshare_flush(struct mmu_gather *tlb, struct vm_area_struct *vma)
+{
+ /*
+ * We must synchronize page table unsharing such that nobody will
+ * try reusing a previously-shared page table while it might still
+ * be in use by previous sharers (TLB, GUP_fast).
+ */
+ i_mmap_assert_write_locked(vma->vm_file->f_mapping);
+
+ tlb_flush_unshared_tables(tlb);
+}
+
#else /* !CONFIG_ARCH_WANT_HUGE_PMD_SHARE */
pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma,
@@ -7174,12 +7186,16 @@ pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma,
return NULL;
}
-int huge_pmd_unshare(struct mm_struct *mm, struct vm_area_struct *vma,
- unsigned long addr, pte_t *ptep)
+int huge_pmd_unshare(struct mmu_gather *tlb, struct vm_area_struct *vma,
+ unsigned long addr, pte_t *ptep)
{
return 0;
}
+void huge_pmd_unshare_flush(struct mmu_gather *tlb, struct vm_area_struct *vma)
+{
+}
+
void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
unsigned long *start, unsigned long *end)
{
@@ -7521,6 +7537,7 @@ static void hugetlb_unshare_pmds(struct vm_area_struct *vma,
unsigned long sz = huge_page_size(h);
struct mm_struct *mm = vma->vm_mm;
struct mmu_notifier_range range;
+ struct mmu_gather tlb;
unsigned long address;
spinlock_t *ptl;
pte_t *ptep;
@@ -7532,6 +7549,8 @@ static void hugetlb_unshare_pmds(struct vm_area_struct *vma,
return;
flush_cache_range(vma, start, end);
+ tlb_gather_mmu_vma(&tlb, vma);
+
/*
* No need to call adjust_range_if_pmd_sharing_possible(), because
* we have already done the PUD_SIZE alignment.
@@ -7550,10 +7569,10 @@ static void hugetlb_unshare_pmds(struct vm_area_struct *vma,
if (!ptep)
continue;
ptl = huge_pte_lock(h, mm, ptep);
- huge_pmd_unshare(mm, vma, address, ptep);
+ huge_pmd_unshare(&tlb, vma, address, ptep);
spin_unlock(ptl);
}
- flush_hugetlb_tlb_range(vma, start, end);
+ huge_pmd_unshare_flush(&tlb, vma);
if (take_locks) {
i_mmap_unlock_write(vma->vm_file->f_mapping);
hugetlb_vma_unlock_write(vma);
@@ -7563,6 +7582,7 @@ static void hugetlb_unshare_pmds(struct vm_area_struct *vma,
* Documentation/mm/mmu_notifier.rst.
*/
mmu_notifier_invalidate_range_end(&range);
+ tlb_finish_mmu(&tlb);
}
/*
diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
index 3a2c3f8cad2f..a6b55fa23fd6 100644
--- a/mm/mmu_gather.c
+++ b/mm/mmu_gather.c
@@ -9,6 +9,7 @@
#include <linux/rcupdate.h>
#include <linux/smp.h>
#include <linux/swap.h>
+#include <linux/hugetlb.h>
#include <asm/pgalloc.h>
#include <asm/tlb.h>
@@ -290,6 +291,7 @@ static void __tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm,
tlb->page_size = 0;
#endif
+ tlb->fully_unshared_tables = 0;
__tlb_reset_range(tlb);
inc_tlb_flush_pending(tlb->mm);
}
@@ -323,6 +325,31 @@ void tlb_gather_mmu_fullmm(struct mmu_gather *tlb, struct mm_struct *mm)
__tlb_gather_mmu(tlb, mm, true);
}
+/**
+ * tlb_gather_mmu_vma - initialize an mmu_gather structure for operating on a
+ * single VMA
+ * @tlb: the mmu_gather structure to initialize
+ * @vma: the vm_area_struct
+ *
+ * Called to initialize an (on-stack) mmu_gather structure for operating on
+ * a single VMA. In contrast to tlb_gather_mmu(), calling this function will
+ * not require another call to tlb_start_vma(). In contrast to tlb_start_vma(),
+ * this function will *not* call flush_cache_range().
+ *
+ * For hugetlb VMAs, this function will also initialize the mmu_gather
+ * page_size accordingly, not requiring a separate call to
+ * tlb_change_page_size().
+ *
+ */
+void tlb_gather_mmu_vma(struct mmu_gather *tlb, struct vm_area_struct *vma)
+{
+ tlb_gather_mmu(tlb, vma->vm_mm);
+ tlb_update_vma_flags(tlb, vma);
+ if (is_vm_hugetlb_page(vma))
+ /* All entries have the same size. */
+ tlb_change_page_size(tlb, huge_page_size(hstate_vma(vma)));
+}
+
/**
* tlb_finish_mmu - finish an mmu_gather structure
* @tlb: the mmu_gather structure to finish
@@ -332,6 +359,12 @@ void tlb_gather_mmu_fullmm(struct mmu_gather *tlb, struct mm_struct *mm)
*/
void tlb_finish_mmu(struct mmu_gather *tlb)
{
+ /*
+ * We expect an earlier huge_pmd_unshare_flush() call to sort this out,
+ * due to complicated locking requirements with page table unsharing.
+ */
+ VM_WARN_ON_ONCE(tlb->fully_unshared_tables);
+
/*
* If there are parallel threads are doing PTE changes on same range
* under non-exclusive lock (e.g., mmap_lock read-side) but defer TLB
diff --git a/mm/rmap.c b/mm/rmap.c
index 49d48a427f08..886000da89a6 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -75,7 +75,7 @@
#include <linux/userfaultfd_k.h>
#include <linux/mm_inline.h>
-#include <asm/tlbflush.h>
+#include <asm/tlb.h>
#define CREATE_TRACE_POINTS
#include <trace/events/tlb.h>
@@ -1561,18 +1561,22 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
* if unsuccessful.
*/
if (!anon) {
+ struct mmu_gather tlb;
+
VM_BUG_ON(!(flags & TTU_RMAP_LOCKED));
if (!hugetlb_vma_trylock_write(vma)) {
page_vma_mapped_walk_done(&pvmw);
ret = false;
break;
}
- if (huge_pmd_unshare(mm, vma, address, pvmw.pte)) {
+
+ tlb_gather_mmu_vma(&tlb, vma);
+ if (huge_pmd_unshare(&tlb, vma, address, pvmw.pte)) {
hugetlb_vma_unlock_write(vma);
- flush_tlb_range(vma,
- range.start, range.end);
+ huge_pmd_unshare_flush(&tlb, vma);
mmu_notifier_invalidate_range(mm,
range.start, range.end);
+ tlb_finish_mmu(&tlb);
/*
* The PMD table was unmapped,
* consequently unmapping the folio.
@@ -1581,6 +1585,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
break;
}
hugetlb_vma_unlock_write(vma);
+ tlb_finish_mmu(&tlb);
}
pteval = huge_ptep_clear_flush(vma, address, pvmw.pte);
} else {
@@ -1945,19 +1950,22 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
* fail if unsuccessful.
*/
if (!anon) {
+ struct mmu_gather tlb;
+
VM_BUG_ON(!(flags & TTU_RMAP_LOCKED));
if (!hugetlb_vma_trylock_write(vma)) {
page_vma_mapped_walk_done(&pvmw);
ret = false;
break;
}
- if (huge_pmd_unshare(mm, vma, address, pvmw.pte)) {
+
+ tlb_gather_mmu_vma(&tlb, vma);
+ if (huge_pmd_unshare(&tlb, vma, address, pvmw.pte)) {
hugetlb_vma_unlock_write(vma);
- flush_tlb_range(vma,
- range.start, range.end);
+ huge_pmd_unshare_flush(&tlb, vma);
mmu_notifier_invalidate_range(mm,
range.start, range.end);
-
+ tlb_finish_mmu(&tlb);
/*
* The PMD table was unmapped,
* consequently unmapping the folio.
@@ -1966,6 +1974,7 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
break;
}
hugetlb_vma_unlock_write(vma);
+ tlb_finish_mmu(&tlb);
}
/* Nuke the hugetlb page table entry */
pteval = huge_ptep_clear_flush(vma, address, pvmw.pte);
--
2.43.0
^ permalink raw reply [flat|nested] 5+ messages in thread