Migration entries do not contribute to a page's reference count: move
__split_huge_pmd_locked()'s page_ref_add() into pmd_migration's else
block (along with the page_count() check - a page is quite likely to
to have reference count frozen to 0 when a migration entry is found).
This will fix a very rare anonymous memory leak, after a split_huge_pmd()
raced with an anon split_huge_page() or an anon THP migrate_pages(): since
the wrongly raised refcount stopped the page (perhaps small, perhaps huge,
depending on when the race hit) from ever being freed. At first I thought
there were worse risks, from prematurely unfreezing a frozen page: but now
think that would only affect page cache pages, which do not come this way
(except for anonymous pages in swap cache, perhaps).
Fixes: ec0abae6dcdf ("mm/thp: fix __split_huge_pmd_locked() for migration PMD")
Signed-off-by: Hugh Dickins <[email protected]>
---
That's an unfair "Fixes": it did not introduce the problem, but it
missed this aspect of the problem; and will be a good guide to where this
refix should go if stable backports are asked for.
mm/huge_memory.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2039,9 +2039,9 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
young = pmd_young(old_pmd);
soft_dirty = pmd_soft_dirty(old_pmd);
uffd_wp = pmd_uffd_wp(old_pmd);
+ VM_BUG_ON_PAGE(!page_count(page), page);
+ page_ref_add(page, HPAGE_PMD_NR - 1);
}
- VM_BUG_ON_PAGE(!page_count(page), page);
- page_ref_add(page, HPAGE_PMD_NR - 1);
/*
* Withdraw the table only after we mark the pmd entry invalid.
On Thu, 3 Mar 2022, Yang Shi wrote:
> On Wed, Mar 2, 2022 at 5:43 PM Hugh Dickins <[email protected]> wrote:
> >
> > Migration entries do not contribute to a page's reference count: move
> > __split_huge_pmd_locked()'s page_ref_add() into pmd_migration's else
> > block (along with the page_count() check - a page is quite likely to
> > to have reference count frozen to 0 when a migration entry is found).
> >
> > This will fix a very rare anonymous memory leak, after a split_huge_pmd()
> > raced with an anon split_huge_page() or an anon THP migrate_pages(): since
> > the wrongly raised refcount stopped the page (perhaps small, perhaps huge,
> > depending on when the race hit) from ever being freed. At first I thought
> > there were worse risks, from prematurely unfreezing a frozen page: but now
> > think that would only affect page cache pages, which do not come this way
> > (except for anonymous pages in swap cache, perhaps).
>
> Thanks for catching this. I agree there may be anon memory leak due to
> bumped refcount. But I don't think it could affect page cache page
> since that code (bumping refcount) is never called for page cache page
> IIUC.
Yes, that's what I meant by unfreezing "would only affect page cache pages,
which do not come this way". But then remembered that anonymous pages in
swap cache also have non-NULL page_mapping(), and involve freezing in the
same way that page cache pages do. Now, offhand I forget the status of
hugepage swap, so maybe that's impossible, hence "perhaps". Easier to
fix than to think through all the ramifications of not fixing, as usual.
>
> The patch looks good to me. Reviewed-by: Yang Shi <[email protected]>
Thanks,
Hugh
On Wed, Mar 2, 2022 at 5:43 PM Hugh Dickins <[email protected]> wrote:
>
> Migration entries do not contribute to a page's reference count: move
> __split_huge_pmd_locked()'s page_ref_add() into pmd_migration's else
> block (along with the page_count() check - a page is quite likely to
> to have reference count frozen to 0 when a migration entry is found).
>
> This will fix a very rare anonymous memory leak, after a split_huge_pmd()
> raced with an anon split_huge_page() or an anon THP migrate_pages(): since
> the wrongly raised refcount stopped the page (perhaps small, perhaps huge,
> depending on when the race hit) from ever being freed. At first I thought
> there were worse risks, from prematurely unfreezing a frozen page: but now
> think that would only affect page cache pages, which do not come this way
> (except for anonymous pages in swap cache, perhaps).
Thanks for catching this. I agree there may be anon memory leak due to
bumped refcount. But I don't think it could affect page cache page
since that code (bumping refcount) is never called for page cache page
IIUC.
The patch looks good to me. Reviewed-by: Yang Shi <[email protected]>
>
> Fixes: ec0abae6dcdf ("mm/thp: fix __split_huge_pmd_locked() for migration PMD")
> Signed-off-by: Hugh Dickins <[email protected]>
> ---
> That's an unfair "Fixes": it did not introduce the problem, but it
> missed this aspect of the problem; and will be a good guide to where this
> refix should go if stable backports are asked for.
>
> mm/huge_memory.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2039,9 +2039,9 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> young = pmd_young(old_pmd);
> soft_dirty = pmd_soft_dirty(old_pmd);
> uffd_wp = pmd_uffd_wp(old_pmd);
> + VM_BUG_ON_PAGE(!page_count(page), page);
> + page_ref_add(page, HPAGE_PMD_NR - 1);
> }
> - VM_BUG_ON_PAGE(!page_count(page), page);
> - page_ref_add(page, HPAGE_PMD_NR - 1);
>
> /*
> * Withdraw the table only after we mark the pmd entry invalid.