On Mon, Jun 19, 2017 at 10:17:29AM +0100, Mark Rutland wrote:
> commit 3c226c637b69104f6b9f1c6ec5b08d7b741b3229 upstream.
>
> In do_huge_pmd_numa_page(), we attempt to handle a migrating thp pmd by
> waiting until the pmd is unlocked before we return and retry. However, we
> can race with migrate_misplaced_transhuge_page():
>
> // do_huge_pmd_numa_page // migrate_misplaced_transhuge_page()
> // Holds 0 refs on page // Holds 2 refs on page
>
> vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
> /* ... */
> if (pmd_trans_migrating(*vmf->pmd)) {
> page = pmd_page(*vmf->pmd);
> spin_unlock(vmf->ptl);
> ptl = pmd_lock(mm, pmd);
> if (page_count(page) != 2)) {
> /* roll back */
> }
> /* ... */
> mlock_migrate_page(new_page, page);
> /* ... */
> spin_unlock(ptl);
> put_page(page);
> put_page(page); // page freed here
> wait_on_page_locked(page);
> goto out;
> }
>
> This can result in the freed page having its waiters flag set
> unexpectedly, which trips the PAGE_FLAGS_CHECK_AT_PREP checks in the page
> alloc/free functions. This has been observed on arm64 KVM guests.
>
> We can avoid this by having do_huge_pmd_numa_page() take a reference on
> the page before dropping the pmd lock, mirroring what we do in
> __migration_entry_wait().
>
> When we hit the race, migrate_misplaced_transhuge_page() will see the
> reference and abort the migration, as it may do today in other cases.
>
> Fixes: b8916634b77bffb2 ("mm: Prevent parallel splits during THP migration")
> Link: http://lkml.kernel.org/r/1497349722-6731-2-git-send-email-will.deacon@xxxxxxx
> Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
> Acked-by: Steve Capper <steve.capper@xxxxxxx>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
> mm/huge_memory.c | 6 ++++++
> 1 file changed, 6 insertions(+)
Thanks for this and the other backports, all now applied.
greg k-h