From: Mauricio Faria de Oliveira <mfo@canonical.com>
To: Yu Zhao <yuzhao@google.com>
Cc: Minchan Kim <minchan@kernel.org>,
"Huang, Ying" <ying.huang@intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
Yang Shi <shy828301@gmail.com>,
Miaohe Lin <linmiaohe@huawei.com>,
linux-mm@kvack.org, linux-block@vger.kernel.org
Subject: Re: [PATCH v3] mm: fix race between MADV_FREE reclaim and blkdev direct IO read
Date: Wed, 2 Feb 2022 18:27:47 -0300 [thread overview]
Message-ID: <CAO9xwp3DNioiVPJNH9w-eXLxfVmTx9jBpOgq9eatpTFJTTg50Q@mail.gmail.com> (raw)
In-Reply-To: <Yfrh9F67ligMDUB7@google.com>
On Wed, Feb 2, 2022 at 4:56 PM Yu Zhao <yuzhao@google.com> wrote:
>
> On Mon, Jan 31, 2022 at 08:02:55PM -0300, Mauricio Faria de Oliveira wrote:
> > Problem:
> > =======
>
> Thanks for the update. A couple of quick questions:
>
> > Userspace might read the zero-page instead of actual data from a
> > direct IO read on a block device if the buffers have been called
> > madvise(MADV_FREE) on earlier (this is discussed below) due to a
> > race between page reclaim on MADV_FREE and blkdev direct IO read.
>
> 1) would page migration be affected as well?
Could you please elaborate on the potential problem you considered?
I checked migrate_pages() -> try_to_migrate() holds the page lock,
thus shouldn't race with shrink_page_list() -> with try_to_unmap()
(where the issue with MADV_FREE is), but maybe I didn't get you
correctly.
>
> > @@ -1599,7 +1599,30 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> >
> > /* MADV_FREE page check */
> > if (!PageSwapBacked(page)) {
> > - if (!PageDirty(page)) {
> > + int ref_count, map_count;
> > +
> > + /*
> > + * Synchronize with gup_pte_range():
> > + * - clear PTE; barrier; read refcount
> > + * - inc refcount; barrier; read PTE
> > + */
> > + smp_mb();
> > +
> > + ref_count = page_count(page);
> > + map_count = page_mapcount(page);
> > +
> > + /*
> > + * Order reads for page refcount and dirty flag;
> > + * see __remove_mapping().
> > + */
> > + smp_rmb();
>
> 2) why does it need to order against __remove_mapping()? It seems to
> me that here (called from the reclaim path) it can't race with
> __remove_mapping() because both lock the page.
I'll improve that comment in v4. The ordering isn't against __remove_mapping(),
but actually because of an issue described in __remove_mapping()'s comments
(something else that doesn't hold the page lock, just has a page reference, that
may clear the page dirty flag then drop the reference; thus check ref,
then dirty).
Hope this clarifies the question.
Thanks!
>
> > + /*
> > + * The only page refs must be from the isolation
> > + * plus one or more rmap's (dropped by discard:).
> > + */
> > + if ((ref_count == 1 + map_count) &&
> > + !PageDirty(page)) {
> > /* Invalidate as we cleared the pte */
> > mmu_notifier_invalidate_range(mm,
> > address, address + PAGE_SIZE);
--
Mauricio Faria de Oliveira
next prev parent reply other threads:[~2022-02-02 21:28 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-31 23:02 [PATCH v3] mm: fix race between MADV_FREE reclaim and blkdev direct IO read Mauricio Faria de Oliveira
2022-01-31 23:43 ` Andrew Morton
2022-02-01 2:23 ` Mauricio Faria de Oliveira
2022-02-02 14:03 ` Christoph Hellwig
2022-02-02 16:29 ` Mauricio Faria de Oliveira
2022-02-02 19:56 ` Yu Zhao
2022-02-02 21:27 ` Mauricio Faria de Oliveira [this message]
2022-02-02 21:53 ` Yu Zhao
2022-02-03 22:17 ` Mauricio Faria de Oliveira
2022-02-04 5:56 ` Yu Zhao
2022-02-04 7:03 ` John Hubbard
2022-02-04 18:59 ` Mauricio Faria de Oliveira
2022-02-04 18:58 ` Mauricio Faria de Oliveira
2022-02-16 6:48 ` Huang, Ying
2022-02-16 21:58 ` Yu Zhao
2022-02-16 22:00 ` Yu Zhao
2022-02-17 6:08 ` Huang, Ying
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=CAO9xwp3DNioiVPJNH9w-eXLxfVmTx9jBpOgq9eatpTFJTTg50Q@mail.gmail.com \
--to=mfo@canonical.com \
--cc=akpm@linux-foundation.org \
--cc=linmiaohe@huawei.com \
--cc=linux-block@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=minchan@kernel.org \
--cc=shy828301@gmail.com \
--cc=ying.huang@intel.com \
--cc=yuzhao@google.com \
/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).