This is a note to let you know that I've just added the patch titled
mm: numa: avoid waiting on freed migrated pages
to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
mm-numa-avoid-waiting-on-freed-migrated-pages.patch
and it can be found in the queue-4.9 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 3c226c637b69104f6b9f1c6ec5b08d7b741b3229 Mon Sep 17 00:00:00 2001
From: Mark Rutland <mark.rutland@xxxxxxx>
Date: Fri, 16 Jun 2017 14:02:34 -0700
Subject: mm: numa: avoid waiting on freed migrated pages
From: Mark Rutland <mark.rutland@xxxxxxx>
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>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
mm/huge_memory.c | 6 ++++++
1 file changed, 6 insertions(+)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1227,8 +1227,11 @@ int do_huge_pmd_numa_page(struct fault_e
*/
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_e
/* 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;
}
Patches currently in stable-queue which might be from mark.rutland@xxxxxxx are
queue-4.9/mm-numa-avoid-waiting-on-freed-migrated-pages.patch