[v2,01/10] mm/thp: fix __split_huge_pmd_locked() on shmem migration entry (fwd)

Commit Message

Hugh Dickins June 9, 2021, 4:05 a.m. UTC

---------- Forwarded message ----------
Date: Tue, 8 Jun 2021 21:00:12 -0700 (PDT)
From: Hugh Dickins <[e-mail]>
To: Andrew Morton <[e-mail]>
Cc: Hugh Dickins <[e-mail]>,
    Kirill A. Shutemov <[e-mail]>,
    Yang Shi <[e-mail]>, Wang Yugui <[e-mail]>,
    Matthew Wilcox <[e-mail]>,
    Naoya Horiguchi <[e-mail]>,
    Alistair Popple <[e-mail]>, Ralph Campbell <[e-mail]>,
    Zi Yan <[e-mail]>, Miaohe Lin <[e-mail]>,
    Minchan Kim <[e-mail]>, Jue Wang <[e-mail]>,
    Peter Xu <[e-mail]>, Jan Kara <[e-mail]>,
    Shakeel Butt <[e-mail]>, Oscar Salvador <[e-mail]>
Subject: [PATCH v2 01/10] mm/thp: fix __split_huge_pmd_locked() on shmem
    migration entry

Stressing huge tmpfs page migration racing hole punch often crashed on the
VM_BUG_ON(!pmd_present) in pmdp_huge_clear_flush(), with DEBUG_VM=y kernel;
or shortly afterwards, on a bad dereference in __split_huge_pmd_locked()
when DEBUG_VM=n.  They forgot to allow for pmd migration entries in the
non-anonymous case.

Full disclosure: those particular experiments were on a kernel with more
relaxed mmap_lock and i_mmap_rwsem locking, and were not repeated on the
vanilla kernel: it is conceivable that stricter locking happens to avoid
those cases, or makes them less likely; but __split_huge_pmd_locked()
already allowed for pmd migration entries when handling anonymous THPs,
so this commit brings the shmem and file THP handling into line.

And while there: use old_pmd rather than _pmd, as in the following blocks;
and make it clearer to the eye that the !vma_is_anonymous() block is
self-contained, making an early return after accounting for unmapping.

Fixes: e71769ae5260 ("mm: enable thp migration for shmem thp")
Signed-off-by: Hugh Dickins <[e-mail]>
Cc: <[e-mail]>
---
v2: omit is_huge_zero_pmd() mods (done differently in next), per Kirill

 mm/huge_memory.c     | 27 ++++++++++++++++++---------
 mm/pgtable-generic.c |  5 ++---
 2 files changed, 20 insertions(+), 12 deletions(-)

Comments

Kirill A . Shutemov June 9, 2021, 10:17 a.m. UTC | #1

On Tue, Jun 08, 2021 at 09:05:21PM -0700, Hugh Dickins wrote:
> 
> 
> ---------- Forwarded message ----------
> Date: Tue, 8 Jun 2021 21:00:12 -0700 (PDT)
> From: Hugh Dickins <[e-mail]>
> To: Andrew Morton <[e-mail]>
> Cc: Hugh Dickins <[e-mail]>,
>     Kirill A. Shutemov <[e-mail]>,
>     Yang Shi <[e-mail]>, Wang Yugui <[e-mail]>,
>     Matthew Wilcox <[e-mail]>,
>     Naoya Horiguchi <[e-mail]>,
>     Alistair Popple <[e-mail]>, Ralph Campbell <[e-mail]>,
>     Zi Yan <[e-mail]>, Miaohe Lin <[e-mail]>,
>     Minchan Kim <[e-mail]>, Jue Wang <[e-mail]>,
>     Peter Xu <[e-mail]>, Jan Kara <[e-mail]>,
>     Shakeel Butt <[e-mail]>, Oscar Salvador <[e-mail]>
> Subject: [PATCH v2 01/10] mm/thp: fix __split_huge_pmd_locked() on shmem
>     migration entry
> 
> Stressing huge tmpfs page migration racing hole punch often crashed on the
> VM_BUG_ON(!pmd_present) in pmdp_huge_clear_flush(), with DEBUG_VM=y kernel;
> or shortly afterwards, on a bad dereference in __split_huge_pmd_locked()
> when DEBUG_VM=n.  They forgot to allow for pmd migration entries in the
> non-anonymous case.
> 
> Full disclosure: those particular experiments were on a kernel with more
> relaxed mmap_lock and i_mmap_rwsem locking, and were not repeated on the
> vanilla kernel: it is conceivable that stricter locking happens to avoid
> those cases, or makes them less likely; but __split_huge_pmd_locked()
> already allowed for pmd migration entries when handling anonymous THPs,
> so this commit brings the shmem and file THP handling into line.
> 
> And while there: use old_pmd rather than _pmd, as in the following blocks;
> and make it clearer to the eye that the !vma_is_anonymous() block is
> self-contained, making an early return after accounting for unmapping.
> 
> Fixes: e71769ae5260 ("mm: enable thp migration for shmem thp")
> Signed-off-by: Hugh Dickins <[e-mail]>
> Cc: <[e-mail]>

Acked-by: Kirill A. Shutemov <[e-mail]>

Yang Shi June 9, 2021, 4:53 p.m. UTC | #2

On Tue, Jun 8, 2021 at 9:05 PM Hugh Dickins <[e-mail]> wrote:
>
>
>
> ---------- Forwarded message ----------
> Date: Tue, 8 Jun 2021 21:00:12 -0700 (PDT)
> From: Hugh Dickins <[e-mail]>
> To: Andrew Morton <[e-mail]>
> Cc: Hugh Dickins <[e-mail]>,
>     Kirill A. Shutemov <[e-mail]>,
>     Yang Shi <[e-mail]>, Wang Yugui <[e-mail]>,
>     Matthew Wilcox <[e-mail]>,
>     Naoya Horiguchi <[e-mail]>,
>     Alistair Popple <[e-mail]>, Ralph Campbell <[e-mail]>,
>     Zi Yan <[e-mail]>, Miaohe Lin <[e-mail]>,
>     Minchan Kim <[e-mail]>, Jue Wang <[e-mail]>,
>     Peter Xu <[e-mail]>, Jan Kara <[e-mail]>,
>     Shakeel Butt <[e-mail]>, Oscar Salvador <[e-mail]>
> Subject: [PATCH v2 01/10] mm/thp: fix __split_huge_pmd_locked() on shmem
>     migration entry
>
> Stressing huge tmpfs page migration racing hole punch often crashed on the
> VM_BUG_ON(!pmd_present) in pmdp_huge_clear_flush(), with DEBUG_VM=y kernel;
> or shortly afterwards, on a bad dereference in __split_huge_pmd_locked()
> when DEBUG_VM=n.  They forgot to allow for pmd migration entries in the
> non-anonymous case.
>
> Full disclosure: those particular experiments were on a kernel with more
> relaxed mmap_lock and i_mmap_rwsem locking, and were not repeated on the
> vanilla kernel: it is conceivable that stricter locking happens to avoid
> those cases, or makes them less likely; but __split_huge_pmd_locked()
> already allowed for pmd migration entries when handling anonymous THPs,
> so this commit brings the shmem and file THP handling into line.
>
> And while there: use old_pmd rather than _pmd, as in the following blocks;
> and make it clearer to the eye that the !vma_is_anonymous() block is
> self-contained, making an early return after accounting for unmapping.
>
> Fixes: e71769ae5260 ("mm: enable thp migration for shmem thp")
> Signed-off-by: Hugh Dickins <[e-mail]>
> Cc: <[e-mail]>

Reviewed-by: Yang Shi <[e-mail]>

> ---
> v2: omit is_huge_zero_pmd() mods (done differently in next), per Kirill
>
>  mm/huge_memory.c     | 27 ++++++++++++++++++---------
>  mm/pgtable-generic.c |  5 ++---
>  2 files changed, 20 insertions(+), 12 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 63ed6b25deaa..42cfefc6e66e 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2044,7 +2044,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>         count_vm_event(THP_SPLIT_PMD);
>
>         if (!vma_is_anonymous(vma)) {
> -               _pmd = pmdp_huge_clear_flush_notify(vma, haddr, pmd);
> +               old_pmd = pmdp_huge_clear_flush_notify(vma, haddr, pmd);
>                 /*
>                  * We are going to unmap this huge page. So
>                  * just go ahead and zap it
> @@ -2053,16 +2053,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>                         zap_deposited_table(mm, pmd);
>                 if (vma_is_special_huge(vma))
>                         return;
> -               page = pmd_page(_pmd);
> -               if (!PageDirty(page) && pmd_dirty(_pmd))
> -                       set_page_dirty(page);
> -               if (!PageReferenced(page) && pmd_young(_pmd))
> -                       SetPageReferenced(page);
> -               page_remove_rmap(page, true);
> -               put_page(page);
> +               if (unlikely(is_pmd_migration_entry(old_pmd))) {
> +                       swp_entry_t entry;
> +
> +                       entry = pmd_to_swp_entry(old_pmd);
> +                       page = migration_entry_to_page(entry);
> +               } else {
> +                       page = pmd_page(old_pmd);
> +                       if (!PageDirty(page) && pmd_dirty(old_pmd))
> +                               set_page_dirty(page);
> +                       if (!PageReferenced(page) && pmd_young(old_pmd))
> +                               SetPageReferenced(page);
> +                       page_remove_rmap(page, true);
> +                       put_page(page);
> +               }
>                 add_mm_counter(mm, mm_counter_file(page), -HPAGE_PMD_NR);
>                 return;
> -       } else if (pmd_trans_huge(*pmd) && is_huge_zero_pmd(*pmd)) {
> +       }
> +
> +       if (pmd_trans_huge(*pmd) && is_huge_zero_pmd(*pmd)) {
>                 /*
>                  * FIXME: Do we want to invalidate secondary mmu by calling
>                  * mmu_notifier_invalidate_range() see comments below inside
> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
> index c2210e1cdb51..4e640baf9794 100644
> --- a/mm/pgtable-generic.c
> +++ b/mm/pgtable-generic.c
> @@ -135,9 +135,8 @@ pmd_t pmdp_huge_clear_flush(struct vm_area_struct *vma, unsigned long address,
>  {
>         pmd_t pmd;
>         VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> -       VM_BUG_ON(!pmd_present(*pmdp));
> -       /* Below assumes pmd_present() is true */
> -       VM_BUG_ON(!pmd_trans_huge(*pmdp) && !pmd_devmap(*pmdp));
> +       VM_BUG_ON(pmd_present(*pmdp) && !pmd_trans_huge(*pmdp) &&
> +                          !pmd_devmap(*pmdp));
>         pmd = pmdp_huge_get_and_clear(vma->vm_mm, address, pmdp);
>         flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
>         return pmd;
> --
> 2.26.2
>
>

Patch

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 63ed6b25deaa..42cfefc6e66e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2044,7 +2044,7 @@  static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 	count_vm_event(THP_SPLIT_PMD);
 
 	if (!vma_is_anonymous(vma)) {
-		_pmd = pmdp_huge_clear_flush_notify(vma, haddr, pmd);
+		old_pmd = pmdp_huge_clear_flush_notify(vma, haddr, pmd);
 		/*
 		 * We are going to unmap this huge page. So
 		 * just go ahead and zap it
@@ -2053,16 +2053,25 @@  static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 			zap_deposited_table(mm, pmd);
 		if (vma_is_special_huge(vma))
 			return;
-		page = pmd_page(_pmd);
-		if (!PageDirty(page) && pmd_dirty(_pmd))
-			set_page_dirty(page);
-		if (!PageReferenced(page) && pmd_young(_pmd))
-			SetPageReferenced(page);
-		page_remove_rmap(page, true);
-		put_page(page);
+		if (unlikely(is_pmd_migration_entry(old_pmd))) {
+			swp_entry_t entry;
+
+			entry = pmd_to_swp_entry(old_pmd);
+			page = migration_entry_to_page(entry);
+		} else {
+			page = pmd_page(old_pmd);
+			if (!PageDirty(page) && pmd_dirty(old_pmd))
+				set_page_dirty(page);
+			if (!PageReferenced(page) && pmd_young(old_pmd))
+				SetPageReferenced(page);
+			page_remove_rmap(page, true);
+			put_page(page);
+		}
 		add_mm_counter(mm, mm_counter_file(page), -HPAGE_PMD_NR);
 		return;
-	} else if (pmd_trans_huge(*pmd) && is_huge_zero_pmd(*pmd)) {
+	}
+
+	if (pmd_trans_huge(*pmd) && is_huge_zero_pmd(*pmd)) {
 		/*
 		 * FIXME: Do we want to invalidate secondary mmu by calling
 		 * mmu_notifier_invalidate_range() see comments below inside
diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index c2210e1cdb51..4e640baf9794 100644
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -135,9 +135,8 @@  pmd_t pmdp_huge_clear_flush(struct vm_area_struct *vma, unsigned long address,
 {
 	pmd_t pmd;
 	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
-	VM_BUG_ON(!pmd_present(*pmdp));
-	/* Below assumes pmd_present() is true */
-	VM_BUG_ON(!pmd_trans_huge(*pmdp) && !pmd_devmap(*pmdp));
+	VM_BUG_ON(pmd_present(*pmdp) && !pmd_trans_huge(*pmdp) &&
+			   !pmd_devmap(*pmdp));
 	pmd = pmdp_huge_get_and_clear(vma->vm_mm, address, pmdp);
 	flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
 	return pmd;