[PATCH v4.9.y] mm: numa: avoid waiting on freed migrated pages

[PATCH v4.9.y] mm: numa: avoid waiting on freed migrated pages

[Mark Rutland]

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@arm.com
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Steve Capper <steve.capper@arm.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
 mm/huge_memory.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index d5b2b75..e7d5db9 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1227,8 +1227,11 @@ int do_huge_pmd_numa_page(struct fault_env *fe, pmd_t pmd)
 	 */
 	if (unlikely(pmd_trans_migrating(*fe->pmd))) {
 		page = pmd_page(*fe->pmd);
+		if (!get_page_unless_zero(page))
+			goto out_unlock;
 		spin_unlock(fe->ptl);
 		wait_on_page_locked(page);
+		put_page(page);
 		goto out;
 	}
 
@@ -1260,8 +1263,11 @@ int do_huge_pmd_numa_page(struct fault_env *fe, pmd_t pmd)
 
 	/* Migration could have started since the pmd_trans_migrating check */
 	if (!page_locked) {
+		if (!get_page_unless_zero(page))
+			goto out_unlock;
 		spin_unlock(fe->ptl);
 		wait_on_page_locked(page);
+		put_page(page);
 		page_nid = -1;
 		goto out;
 	}
-- 
1.9.1

Re: [PATCH v4.9.y] mm: numa: avoid waiting on freed migrated pages

[Greg KH]

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@arm.com
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Will Deacon <will.deacon@arm.com>
> Acked-by: Steve Capper <steve.capper@arm.com>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
>  mm/huge_memory.c | 6 ++++++
>  1 file changed, 6 insertions(+)

Thanks for this and the other backports, all now applied.

greg k-h