[PATCH v2 27/32] mm/khugepaged: allow pte_offset_map[_lock]() to fail

Hugh Dickins
2023-06-09T01:42:48Z

__collapse_huge_page_swapin(): don't drop the map after every pte, it
only has to be dropped by do_swap_page(); give up if pte_offset_map()
fails; trace_mm_collapse_huge_page_swapin() at the end, with result;
fix comment on returned result; fix vmf.pgoff, though it's not used.

collapse_huge_page(): use pte_offset_map_lock() on the _pmd returned
from clearing; allow failure, but it should be impossible there.
hpage_collapse_scan_pmd() and collapse_pte_mapped_thp() allow for
pte_offset_map_lock() failure.

Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
---
mm/khugepaged.c | 72 +++++++++++++++++++++++++++++++++----------------
1 file changed, 49 insertions(+), 23 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 732f9ac393fc..49cfa7cdfe93 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -993,9 +993,8 @@ static int check_pmd_still_valid(struct mm_struct *mm,
* Only done if hpage_collapse_scan_pmd believes it is worthwhile.
*
* Called and returns without pte mapped or spinlocks held.
- * Note that if false is returned, mmap_lock will be released.
+ * Returns result: if not SCAN_SUCCEED, mmap_lock has been released.
*/
-
static int __collapse_huge_page_swapin(struct mm_struct *mm,
struct vm_area_struct *vma,
unsigned long haddr, pmd_t *pmd,
@@ -1004,23 +1003,35 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
int swapped_in = 0;
vm_fault_t ret = 0;
unsigned long address, end = haddr + (HPAGE_PMD_NR * PAGE_SIZE);
+ int result;
+ pte_t *pte = NULL;

for (address = haddr; address < end; address += PAGE_SIZE) {
struct vm_fault vmf = {
.vma = vma,
.address = address,
- .pgoff = linear_page_index(vma, haddr),
+ .pgoff = linear_page_index(vma, address),
.flags = FAULT_FLAG_ALLOW_RETRY,
.pmd = pmd,
};

- vmf.pte = pte_offset_map(pmd, address);
- vmf.orig_pte = *vmf.pte;
- if (!is_swap_pte(vmf.orig_pte)) {
- pte_unmap(vmf.pte);
- continue;
+ if (!pte++) {
+ pte = pte_offset_map(pmd, address);
+ if (!pte) {
+ mmap_read_unlock(mm);
+ result = SCAN_PMD_NULL;
+ goto out;
+ }
}
+
+ vmf.orig_pte = *pte;
+ if (!is_swap_pte(vmf.orig_pte))
+ continue;
+
+ vmf.pte = pte;
ret = do_swap_page(&vmf);
+ /* Which unmaps pte (after perhaps re-checking the entry) */
+ pte = NULL;

/*
* do_swap_page returns VM_FAULT_RETRY with released mmap_lock.
@@ -1029,24 +1040,29 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
* resulting in later failure.
*/
if (ret & VM_FAULT_RETRY) {
- trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
/* Likely, but not guaranteed, that page lock failed */
- return SCAN_PAGE_LOCK;
+ result = SCAN_PAGE_LOCK;
+ goto out;
}
if (ret & VM_FAULT_ERROR) {
mmap_read_unlock(mm);
- trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
- return SCAN_FAIL;
+ result = SCAN_FAIL;
+ goto out;
}
swapped_in++;
}

+ if (pte)
+ pte_unmap(pte);
+
/* Drain LRU add pagevec to remove extra pin on the swapped in pages */
if (swapped_in)
lru_add_drain();

- trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 1);
- return SCAN_SUCCEED;
+ result = SCAN_SUCCEED;
+out:
+ trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, result);
+ return result;
}

static int alloc_charge_hpage(struct page **hpage, struct mm_struct *mm,
@@ -1146,9 +1162,6 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
address + HPAGE_PMD_SIZE);
mmu_notifier_invalidate_range_start(&range);

- pte = pte_offset_map(pmd, address);
- pte_ptl = pte_lockptr(mm, pmd);
-
pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
/*
* This removes any huge TLB entry from the CPU so we won't allow
@@ -1163,13 +1176,18 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
mmu_notifier_invalidate_range_end(&range);
tlb_remove_table_sync_one();

- spin_lock(pte_ptl);
- result = __collapse_huge_page_isolate(vma, address, pte, cc,
- &compound_pagelist);
- spin_unlock(pte_ptl);
+ pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
+ if (pte) {
+ result = __collapse_huge_page_isolate(vma, address, pte, cc,
+ &compound_pagelist);
+ spin_unlock(pte_ptl);
+ } else {
+ result = SCAN_PMD_NULL;
+ }

if (unlikely(result != SCAN_SUCCEED)) {
- pte_unmap(pte);
+ if (pte)
+ pte_unmap(pte);
spin_lock(pmd_ptl);
BUG_ON(!pmd_none(*pmd));
/*
@@ -1253,6 +1271,11 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
memset(cc->node_load, 0, sizeof(cc->node_load));
nodes_clear(cc->alloc_nmask);
pte = pte_offset_map_lock(mm, pmd, address, &ptl);
+ if (!pte) {
+ result = SCAN_PMD_NULL;
+ goto out;
+ }
+
for (_address = address, _pte = pte; _pte < pte + HPAGE_PMD_NR;
_pte++, _address += PAGE_SIZE) {
pte_t pteval = *_pte;
@@ -1622,8 +1645,10 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
* lockless_pages_from_mm() and the hardware page walker can access page
* tables while all the high-level locks are held in write mode.
*/
- start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl);
result = SCAN_FAIL;
+ start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl);
+ if (!start_pte)
+ goto drop_immap;

/* step 1: check all mapped PTEs are to the right huge page */
for (i = 0, addr = haddr, pte = start_pte;
@@ -1697,6 +1722,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,

abort:
pte_unmap_unlock(start_pte, ptl);
+drop_immap:
i_mmap_unlock_write(vma->vm_file->f_mapping);
goto drop_hpage;
}
--
2.35.3