linux-kernel - Re: [PATCH] mm, page_alloc: Always use a captured page regardless of compaction result

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Date:   Fri, 19 Apr 2019 14:30:00 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Li Wang <liwang@...hat.com>, linux-mm <linux-mm@...ck.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm, page_alloc: Always use a captured page regardless of
 compaction result

On Fri, Apr 19, 2019 at 02:54:54PM +0200, Vlastimil Babka wrote:
> On 4/19/19 10:51 AM, Mel Gorman wrote:
> > 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
> > occuring when testing OOM handling. Note that the warning is slightly
> > edited for clarity.
> > 
> > [ 1422.124060] WARNING: CPU: 0 PID: 9783 at mm/page_alloc.c:3777 __alloc_pages_direct_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@...hat.com>
> > Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> 
> Ah, noticed the new formal resend only after replying to the first one,
> so here goes again:
> 
> Yup, no need for a Cc: stable on a very rare WARN_ON_ONCE. So the AI
> will pick it anyway...
> 
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> 

With luck, this will be picked up and sent to Linus before 5.1 releases
and then the stable bot will not need to touch the commit at all.

-- 
Mel Gorman
SUSE Labs