linux-mm.kvack.org archive mirror
 help / color / mirror / Atom feed
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Qian Cai <cai@lca.pw>
Cc: Hugh Dickins <hughd@google.com>,
	Matthew Wilcox <willy@infradead.org>,
	 "Kirill A . Shutemov" <kirill@shutemov.name>,
	Linux-MM <linux-mm@kvack.org>,
	 Andrew Morton <akpm@linux-foundation.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	 Amir Goldstein <amir73il@gmail.com>,
	Vivek Goyal <vgoyal@redhat.com>
Subject: Re: [PATCH 0/4] Some more lock_page work..
Date: Wed, 14 Oct 2020 19:44:16 -0700	[thread overview]
Message-ID: <CAHk-=wh9Eu-gNHzqgfvUAAiO=vJ+pWnzxkv+tX55xhGPFy+cOw@mail.gmail.com> (raw)
In-Reply-To: <4794a3fa3742a5e84fb0f934944204b55730829b.camel@lca.pw>

On Wed, Oct 14, 2020 at 6:48 PM Qian Cai <cai@lca.pw> wrote:
>
> While on this topic, I just want to bring up a bug report that we are chasing an
> issue that a process is stuck in the loop of wait_on_page_bit_common() for more
> than 10 minutes before I gave up.

Judging by call trace, that looks like a deadlock rather than a missed wakeup.

The trace isn't reliable, but I find it suspicious that the call trace
just before the fault contains that
"iov_iter_copy_from_user_atomic()".

IOW, I think you're in fuse_fill_write_pages(), which has allocated
the page, locked it, and then it takes a page fault.

And the page fault waits on a page that is locked.

This is a classic deadlock.

The *intent* is that iov_iter_copy_from_user_atomic() returns zero,
and you retry without the page lock held.

HOWEVER.

That's not what fuse actually does. Fuse will do multiple pages, and
it will unlock only the _last_ page. It keeps the other pages locked,
and puts them in an array:

                ap->pages[ap->num_pages] = page;

And after the iov_iter_copy_from_user_atomic() fails, it does that
"unlock" and repeat.

But while the _last_ page was unlocked, the *previous* pages are still
locked in that array. Deadlock.

I really don't think this has anything at all to do with page locking,
and everything to do with fuse_fill_write_pages() having a deadlock if
the source of data is a mmap of one of the pages it is trying to write
to (just with an offset, so that it's not the last page).

See a similar code sequence in generic_perform_write(), but notice how
that code only has *one* page that it locks, and never holds an array
of pages around over that iov_iter_fault_in_readable() thing.

              Linus


  parent reply	other threads:[~2020-10-15  2:44 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-13 19:59 Linus Torvalds
2020-10-13 20:03 ` Linus Torvalds
2020-10-14 13:05   ` Kirill A. Shutemov
2020-10-14 16:53     ` Linus Torvalds
2020-10-14 18:15       ` Matthew Wilcox
2020-10-15 10:41         ` Kirill A. Shutemov
2020-10-15  9:43       ` Kirill A. Shutemov
2020-10-15 16:44         ` Linus Torvalds
2020-10-14  5:50 ` Hugh Dickins
     [not found] ` <4794a3fa3742a5e84fb0f934944204b55730829b.camel@lca.pw>
2020-10-15  2:44   ` Linus Torvalds [this message]
2020-10-15 15:16     ` Possible deadlock in fuse write path (Was: Re: [PATCH 0/4] Some more lock_page work..) Vivek Goyal
2020-10-15 19:55       ` Vivek Goyal
2020-10-15 21:21         ` Linus Torvalds
2020-10-16 10:02           ` Miklos Szeredi
2020-10-16 12:27             ` Matthew Wilcox
2020-10-20 20:42             ` Vivek Goyal
2020-10-21  7:40               ` Miklos Szeredi
2020-10-21 20:12                 ` Vivek Goyal
2020-10-28 20:29                   ` Miklos Szeredi
2021-02-09 10:01                     ` Miklos Szeredi
2021-02-09 19:09                       ` Vivek Goyal
2020-10-16 18:19           ` Vivek Goyal
2020-10-16 18:24             ` Linus Torvalds
2020-10-16 18:24               ` Linus Torvalds
2020-10-16 23:03             ` Dave Chinner

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='CAHk-=wh9Eu-gNHzqgfvUAAiO=vJ+pWnzxkv+tX55xhGPFy+cOw@mail.gmail.com' \
    --to=torvalds@linux-foundation.org \
    --cc=akpm@linux-foundation.org \
    --cc=amir73il@gmail.com \
    --cc=cai@lca.pw \
    --cc=hughd@google.com \
    --cc=kirill@shutemov.name \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=vgoyal@redhat.com \
    --cc=willy@infradead.org \
    --subject='Re: [PATCH 0/4] Some more lock_page work..' \
    /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

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).