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From: Linus Torvalds <>
To: Qian Cai <>
Cc: Hugh Dickins <>,
	Matthew Wilcox <>,
	 "Kirill A . Shutemov" <>,
	Linux-MM <>,
	 Andrew Morton <>,
	linux-fsdevel <>,
	 Amir Goldstein <>,
	Vivek Goyal <>
Subject: Re: [PATCH 0/4] Some more lock_page work..
Date: Wed, 14 Oct 2020 19:44:16 -0700	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Wed, Oct 14, 2020 at 6:48 PM Qian Cai <> 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

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.


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.


  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 [PATCH 0/4] Some more lock_page work 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] ` <>
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

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