MADV_FREE is broken

[Artem Savkov]

Hi All,

We recently started noticing madvise09[1] test from ltp failing strangely. The
test does the following: maps 32 pages, sets MADV_FREE for the range it got,
dirties 2 of the pages, creates memory pressure and check that nondirty pages
are free. The test hanged while accessing the last 4 pages(29-32) of madvised
range at line 121 [2]. Any other process (gdb/cat) accessing those pages
would also hang as would rebooting the machine. It doesn't trigger any debug
warnings or kasan.

The issue bisected to "802a3a92ad7a mm: reclaim MADV_FREE pages" (so 4.12 and
up are affected).

I did some poking around and found out that the "bad" pages had SwapBacked flag
set in shrink_page_list() which confused it a lot. It looks like
mark_page_lazyfree() only calls lru_lazyfree_fn() when the pagevec is full
(that is in batches of 14) and never drains the rest (so last four in madvise09
case).

The patch below greatly reduces the failure rate, but doesn't fix it
completely, it still shows up with the same symptoms (hanging trying to access
last 4 pages) after a bunch of retries.

[1] https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/syscalls/madvise/madvise09.c
[2] https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/syscalls/madvise/madvise09.c#L121

diff --git a/mm/madvise.c b/mm/madvise.c
index 21261ff0466f..a0b868e8b7d2 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -453,6 +453,7 @@ static void madvise_free_page_range(struct mmu_gather *tlb,
 
 	tlb_start_vma(tlb, vma);
 	walk_page_range(addr, end, &free_walk);
+	lru_add_drain();
 	tlb_end_vma(tlb, vma);
 }
 

-- 
Regards,
  Artem