The patch titled
Subject: mm/z3fold.c: use kref to prevent page free/compact race
has been added to the -mm tree. Its filename is
z3fold-use-kref-to-prevent-page-free-compact-race.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/z3fold-use-kref-to-prevent-page-free-compact-race.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/z3fold-use-kref-to-prevent-page-free-compact-race.patch
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------------------------------------------------------
From: Vitaly Wool <vitalywool@xxxxxxxxx>
Subject: mm/z3fold.c: use kref to prevent page free/compact race
There is a race in the current z3fold implementation between do_compact()
called in a work queue context and the page release procedure when page's
kref goes to 0. do_compact() may be waiting for page lock, which is
released by release_z3fold_page_locked right before putting the page onto
the "stale" list, and then the page may be freed as do_compact() modifies
its contents.
The mechanism currently implemented to handle that (checking the
PAGE_STALE flag) is not reliable enough. Instead, we'll use page's kref
counter to guarantee that the page is not released if its compaction is
scheduled. It then becomes compaction function's responsibility to
decrease the counter and quit immediately if the page was actually freed.
Link: http://lkml.kernel.org/r/20171117092032.00ea56f42affbed19f4fcc6c@xxxxxxxxx
Signed-off-by: Vitaly Wool <vitaly.wool@xxxxxxxxxxxxxx>
Cc: <Oleksiy.Avramchenko@xxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/z3fold.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)
diff -puN mm/z3fold.c~z3fold-use-kref-to-prevent-page-free-compact-race mm/z3fold.c
--- a/mm/z3fold.c~z3fold-use-kref-to-prevent-page-free-compact-race
+++ a/mm/z3fold.c
@@ -404,8 +404,7 @@ static void do_compact_page(struct z3fol
WARN_ON(z3fold_page_trylock(zhdr));
else
z3fold_page_lock(zhdr);
- if (test_bit(PAGE_STALE, &page->private) ||
- !test_and_clear_bit(NEEDS_COMPACTING, &page->private)) {
+ if (WARN_ON(!test_and_clear_bit(NEEDS_COMPACTING, &page->private))) {
z3fold_page_unlock(zhdr);
return;
}
@@ -413,6 +412,11 @@ static void do_compact_page(struct z3fol
list_del_init(&zhdr->buddy);
spin_unlock(&pool->lock);
+ if (kref_put(&zhdr->refcount, release_z3fold_page_locked)) {
+ atomic64_dec(&pool->pages_nr);
+ return;
+ }
+
z3fold_compact_page(zhdr);
unbuddied = get_cpu_ptr(pool->unbuddied);
fchunks = num_free_chunks(zhdr);
@@ -753,9 +757,11 @@ static void z3fold_free(struct z3fold_po
list_del_init(&zhdr->buddy);
spin_unlock(&pool->lock);
zhdr->cpu = -1;
+ kref_get(&zhdr->refcount);
do_compact_page(zhdr, true);
return;
}
+ kref_get(&zhdr->refcount);
queue_work_on(zhdr->cpu, pool->compact_wq, &zhdr->work);
z3fold_page_unlock(zhdr);
}
_
Patches currently in -mm which might be from vitalywool@xxxxxxxxx are
z3fold-use-kref-to-prevent-page-free-compact-race.patch