Re: v5.1-rc5 s390x WARNING

[Mel Gorman <mgorman@techsingularity.net>]

On Wed, Apr 17, 2019 at 10:54:38AM +0200, Vlastimil Babka wrote:
> On 4/17/19 10:35 AM, Li Wang wrote:
> > Hi there,
> > 
> > I catched this warning on v5.1-rc5(s390x). It was trggiered in fork & malloc & memset stress test, but the reproduced rate is very low. I'm working on find a stable reproducer for it. 
> > 
> > Anyone can have a look first?
> > 
> > [ 1422.124060] WARNING: CPU: 0 PID: 9783 at mm/page_alloc.c:3777 __alloc_pages_irect_compact+0x182/0x190
> 
> This means compaction was either skipped or deferred, yet it captured a
> page. We have some registers with value 1 and 2, which is
> COMPACT_SKIPPED and COMPACT_DEFERRED, so it could be one of those.
> Probably COMPACT_SKIPPED. I think a race is possible:
> 
> - compact_zone_order() sets up current->capture_control
> - compact_zone() calls compaction_suitable() which returns
> COMPACT_SKIPPED, so it also returns
> - interrupt comes and its processing happens to free a page that forms
> high-order page, since 'current' isn't changed during interrupt (IIRC?)
> the capture_control is still active and the page is captured
> - compact_zone_order() does *capture = capc.page
> 
> What do you think, Mel, does it look plausible?

It's plausible, just extremely unlikely. I think the most likely result
was that a page filled the per-cpu lists and a bunch of pages got freed
in a batch from interrupt context.

> Not sure whether we want
> to try avoiding this scenario, or just remove the warning and be
> grateful for the successful capture :)
> 

Avoiding the scenario is pointless because it's not wrong. The check was
initially meant to catch serious programming errors such as using a
stale page pointer so I think the right patch is below. Li Wang, how
reproducible is this and would you be willing to test it?

---8<---
mm, page_alloc: Always use a captured page regardless of compaction result

During the development of commit 5e1f0f098b46 ("mm, compaction: capture
a page under direct compaction"), a paranoid check was added to ensure
that if a captured page was available after compaction that it was
consistent with the final state of compaction. The intent was to catch
serious programming bugs such as using a stale page pointer and causing
corruption problems.

However, it is possible to get a captured page even if compaction was
unsuccessful if an interrupt triggered and happened to free pages in
interrupt context that got merged into a suitable high-order page. It's
highly unlikely but Li Wang did report the following warning on s390

[ 1422.124060] WARNING: CPU: 0 PID: 9783 at mm/page_alloc.c:3777 __alloc_pages_irect_compact+0x182/0x190
[ 1422.124065] Modules linked in: rpcsec_gss_krb5 auth_rpcgss nfsv4 dns_resolver
 nfs lockd grace fscache sunrpc pkey ghash_s390 prng xts aes_s390 des_s390
 des_generic sha512_s390 zcrypt_cex4 zcrypt vmur binfmt_misc ip_tables xfs
 libcrc32c dasd_fba_mod qeth_l2 dasd_eckd_mod dasd_mod qeth qdio lcs ctcm
 ccwgroup fsm dm_mirror dm_region_hash dm_log dm_mod
[ 1422.124086] CPU: 0 PID: 9783 Comm: copy.sh Kdump: loaded Not tainted 5.1.0-rc 5 #1

This patch simply removes the check entirely instead of trying to be
clever about pages freed from interrupt context. If a serious programming
error was introduced, it is highly likely to be caught by prep_new_page()
instead.

Fixes: 5e1f0f098b46 ("mm, compaction: capture a page under direct compaction")
Reported-by: Li Wang <liwang@redhat.com>
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
 mm/page_alloc.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d96ca5bc555b..cfaba3889fa2 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3773,11 +3773,6 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
 	memalloc_noreclaim_restore(noreclaim_flag);
 	psi_memstall_leave(&pflags);
 
-	if (*compact_result <= COMPACT_INACTIVE) {
-		WARN_ON_ONCE(page);
-		return NULL;
-	}
-
 	/*
 	 * At least in one zone compaction wasn't deferred or skipped, so let's
 	 * count a compaction stall