From: David Rientjes <rientjes@google.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Vlastimil Babka <vbabka@suse.cz>,
Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
Mel Gorman <mgorman@techsingularity.net>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [patch] mm, page_alloc: move_freepages should not examine struct page of reserved memory
Date: Tue, 13 Aug 2019 16:31:35 -0700 (PDT) [thread overview]
Message-ID: <alpine.DEB.2.21.1908131625310.224017@chino.kir.corp.google.com> (raw)
In-Reply-To: <20190813141630.bd8cee48e6a83ca77eead6ad@linux-foundation.org>
On Tue, 13 Aug 2019, Andrew Morton wrote:
> > After commit 907ec5fca3dc ("mm: zero remaining unavailable struct pages"),
> > struct page of reserved memory is zeroed. This causes page->flags to be 0
> > and fixes issues related to reading /proc/kpageflags, for example, of
> > reserved memory.
> >
> > The VM_BUG_ON() in move_freepages_block(), however, assumes that
> > page_zone() is meaningful even for reserved memory. That assumption is no
> > longer true after the aforementioned commit.
> >
> > There's no reason why move_freepages_block() should be testing the
> > legitimacy of page_zone() for reserved memory; its scope is limited only
> > to pages on the zone's freelist.
> >
> > Note that pfn_valid() can be true for reserved memory: there is a backing
> > struct page. The check for page_to_nid(page) is also buggy but reserved
> > memory normally only appears on node 0 so the zeroing doesn't affect this.
> >
> > Move the debug checks to after verifying PageBuddy is true. This isolates
> > the scope of the checks to only be for buddy pages which are on the zone's
> > freelist which move_freepages_block() is operating on. In this case, an
> > incorrect node or zone is a bug worthy of being warned about (and the
> > examination of struct page is acceptable bcause this memory is not
> > reserved).
>
> I'm thinking Fixes:907ec5fca3dc and Cc:stable? But 907ec5fca3dc is
> almost a year old, so you were doing something special to trigger this?
>
We noticed it almost immediately after bringing 907ec5fca3dc in on
CONFIG_DEBUG_VM builds. It depends on finding specific free pages in the
per-zone free area where the math in move_freepages() will bring the start
or end pfn into reserved memory and wanting to claim that entire pageblock
as a new migratetype. So the path will be rare, require CONFIG_DEBUG_VM,
and require fallback to a different migratetype.
Some struct pages were already zeroed from reserve pages before
907ec5fca3c so it theoretically could trigger before this commit. I think
it's rare enough under a config option that most people don't run that
others may not have noticed. I wouldn't argue against a stable tag and
the backport should be easy enough, but probably wouldn't single out a
commit that this is fixing.
next prev parent reply other threads:[~2019-08-13 23:31 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-13 3:37 [patch] mm, page_alloc: move_freepages should not examine struct page of reserved memory David Rientjes
2019-08-13 13:03 ` Vlastimil Babka
2019-08-13 17:22 ` David Rientjes
2019-08-14 7:42 ` Vlastimil Babka
2019-08-13 21:16 ` Andrew Morton
2019-08-13 23:31 ` David Rientjes [this message]
2019-08-14 22:49 ` Andrew Morton
2019-08-19 13:35 ` Mel Gorman
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=alpine.DEB.2.21.1908131625310.224017@chino.kir.corp.google.com \
--to=rientjes@google.com \
--cc=akpm@linux-foundation.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mgorman@techsingularity.net \
--cc=n-horiguchi@ah.jp.nec.com \
--cc=vbabka@suse.cz \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).