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owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/9/2025 6:14 PM, Harry Yoo wrote: > On Tue, Sep 09, 2025 at 12:43:57PM -0600, Jane Chu wrote: >> commit 59d9094df3d79 introduced ->pt_share_count dedicated to > > nit: the format should be: > commit ("summary") > ? > >> hugetlb PMD share count tracking, but omitted fixing >> copy_hugetlb_page_range(), leaving the function relying on >> page_count() for tracking that no longer works. >> >> When lazy page table copy for hugetlb is disabled (commit bcd51a3c679d), > > same here. > >> fork()'ing with hugetlb PMD sharing quickly lockup - >> >> [ 239.446559] watchdog: BUG: soft lockup - CPU#75 stuck for 27s! >> [ 239.446611] RIP: 0010:native_queued_spin_lock_slowpath+0x7e/0x2e0 >> [ 239.446631] Call Trace: >> [ 239.446633] >> [ 239.446636] _raw_spin_lock+0x3f/0x60 >> [ 239.446639] copy_hugetlb_page_range+0x258/0xb50 >> [ 239.446645] copy_page_range+0x22b/0x2c0 >> [ 239.446651] dup_mmap+0x3e2/0x770 >> [ 239.446654] dup_mm.constprop.0+0x5e/0x230 >> [ 239.446657] copy_process+0xd17/0x1760 >> [ 239.446660] kernel_clone+0xc0/0x3e0 >> [ 239.446661] __do_sys_clone+0x65/0xa0 >> [ 239.446664] do_syscall_64+0x82/0x930 >> [ 239.446668] ? count_memcg_events+0xd2/0x190 >> [ 239.446671] ? syscall_trace_enter+0x14e/0x1f0 >> [ 239.446676] ? syscall_exit_work+0x118/0x150 >> [ 239.446677] ? arch_exit_to_user_mode_prepare.constprop.0+0x9/0xb0 >> [ 239.446681] ? clear_bhb_loop+0x30/0x80 >> [ 239.446684] ? clear_bhb_loop+0x30/0x80 >> [ 239.446686] entry_SYSCALL_64_after_hwframe+0x76/0x7e >> >> There are two options to resolve the potential latent issue: >> 1. remove the PMD sharing awareness from copy_hugetlb_page_range(), >> 2. fix it. >> This patch opts for the second option. >> >> Fixes: 59d9094df3d79 ("mm: hugetlb: independent PMD page table shared >> count") > > nit: we don't add newline even when Fixes: tag is line is over 75 characters? > >> Signed-off-by: Jane Chu >> --- > > The change in general looks good to me, > Reviewed-by: Harry Yoo > > +Cc Jann Horn who backported the commit 59d9094df3d79. Thanks Harry! Will fix the commit message and send v2.> > Elaborating a little bit why it doesn't need to be backported: > TL;DR: One needs to backport commit 3aa4ed8040e15 to pre-v6.0 kernels, > or revert commit bcd51a3c679d to trigger this. > > commit 59d9094df3d79 ("mm: hugetlb: independent PMD page table shared count") > is introduced in 6.13 and backported to 5.10.y, 5.15.y, 6.1.y, 6.6.y, 6.12.y. > > As lazy page table copy is enabled in v6.0 by commit bcd51a3c679d > ("hugetlb: lazy page table copies in fork()"), the bug is not triggered > because shared hugetlb VMAs go through lazy page table copy logic > (vma_needs_copy() returns false) or they can't share page tables > (uffd_disable_huge_pmd_share() returns false). > > They shouldn't have anon_vma, VM_PFNMAP, VM_MIXEDMAP and UFFD-WP VMA > cannot share page tables - so either vma_needs_copy() return false, or > page tables cannot be shared. > > And before commit 3aa4ed8040e15 ("mm/hugetlb: make detecting shared pte > more reliable") introduced in v6.1, copy_hugetlb_page_range() doesn't check > refcount to determine whether the page table is shared. Yes, it's the combination of v6.1: 3aa4ed8040e15 mm/hugetlb: make detecting shared pte more reliable v6.13: 59d9094df3d79 mm: hugetlb: independent PMD page table shared count without v6.0: bcd51a3c679d hugetlb: lazy page table copies in fork() that is problematic. Since the problematic combination doesn't exist in any upstream release, no need to backport. The patch is for a potential latent bug. thanks, -jane > >> mm/hugetlb.c | 13 ++++--------- >> 1 file changed, 4 insertions(+), 9 deletions(-) >> >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >> index 753f99b4c718..8ca5b4f7805f 100644 >> --- a/mm/hugetlb.c >> +++ b/mm/hugetlb.c >> @@ -5594,18 +5594,13 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, >> break; >> } >> >> - /* >> - * If the pagetables are shared don't copy or take references. >> - * >> - * dst_pte == src_pte is the common case of src/dest sharing. >> - * However, src could have 'unshared' and dst shares with >> - * another vma. So page_count of ptep page is checked instead >> - * to reliably determine whether pte is shared. >> - */ >> - if (page_count(virt_to_page(dst_pte)) > 1) { >> +#ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING >> + /* If the pagetables are shared don't copy or take references. */ >> + if (ptdesc_pmd_pts_count(virt_to_ptdesc(dst_pte)) > 0) { >> addr |= last_addr_mask; >> continue; >> } >> +#endif >> >> dst_ptl = huge_pte_lock(h, dst, dst_pte); >> src_ptl = huge_pte_lockptr(h, src, src_pte); >> -- >> 2.43.5 >> >