z3fold: fix stale list handling

Fix the situation when clear_bit() is called for page->private before the page pointer is actually assigned. While at it, remove work_busy() check because it is costly and does not give 100% guarantee anyway.

---
 mm/z3fold.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

index b04fa3ba1bf2..b2ba2ba585f3 100644
@@ -250,6 +250,7 @@ static void __release_z3fold_page(struct z3fold_header *zhdr, bool locked)
 
 	WARN_ON(!list_empty(&zhdr->buddy));
 	set_bit(PAGE_STALE, &page->private);
+	clear_bit(NEEDS_COMPACTING, &page->private);
 	spin_lock(&pool->lock);
 	if (!list_empty(&page->lru))
 		list_del(&page->lru);

@@ -303,7 +304,6 @@ static void free_pages_work(struct work_struct *w)
 		list_del(&zhdr->buddy);
 		if (WARN_ON(!test_bit(PAGE_STALE, &page->private)))
 			continue;
-		clear_bit(NEEDS_COMPACTING, &page->private);
 		spin_unlock(&pool->stale_lock);
 		cancel_work_sync(&zhdr->work);
 		free_z3fold_page(page);

@@ -624,10 +624,8 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
 	 * stale pages list. cancel_work_sync() can sleep so we must make
 	 * sure it won't be called in case we're in atomic context.
 	 */
-	if (zhdr && (can_sleep || !work_pending(&zhdr->work) ||
-	    !unlikely(work_busy(&zhdr->work)))) {
+	if (zhdr && (can_sleep || !work_pending(&zhdr->work))) {
 		list_del(&zhdr->buddy);
-		clear_bit(NEEDS_COMPACTING, &page->private);
 		spin_unlock(&pool->stale_lock);
 		if (can_sleep)
 			cancel_work_sync(&zhdr->work);

2.11.0

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Does this fix https://bugzilla.kernel.org/show_bug.cgi?id=196877 ? If so, the bugzilla references and a reported-by should be added.

What are the end-user visible effects of the bug? Please always include this info when fixing bugs.

Should this fix be backported into -stable kernels?

Hi Andrew,

2017-09-14 23:15 GMT+02:00 Andrew Morton <[email protected]>:

I wish it did but it doesn't. The bug you are referring to happens with the "unbuddied" list, and the current version of z3fold_reclaim_page() just doesn't have that code.
This patch fixes the processing of "stale" lists, with stale lists having been introduced with the per-CPU unbuddied lists patch, which is pretty recent.
To fix https://bugzilla.kernel.org/show_bug.cgi?id=196877, we'll have to either backport per-CPU unbuddied lists plus the two fixes, or propose a separate fix.

If page is NULL, clear_bit for page->private will result in a kernel crash.

No, this patch fixes the code that is not in any released kernel yet.

~vitaly

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