From: Vincent Whitchurch <vincent.whitchurch@axis.com>
To: <jack@suse.cz>
Cc: <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>
Subject: __buffer_migrate_page() vs ll_rw_block()
Date: Tue, 1 Jun 2021 15:23:17 +0200 [thread overview]
Message-ID: <20210601132316.GA27976@axis.com> (raw)
I'm seeing occasional squashfs read failures ("squashfs_read_data failed
to read block") when compaction is run at the same time as reading from
squashfs, with something like the commands below. The kernel version is
the latest stable/v5.4 kernel, v5.4.123.
while :; do echo 1 > /proc/sys/vm/compact_memory; done &
while :; do echo 3 > /proc/sys/vm/drop_caches; find fs/ > /dev/null; done &
On this kernel, squashfs uses ll_rw_block(). The problem is that
ll_rw_block() ignores BHs which it can't get a lock on, but
__buffer_migrate_page() can take the lock on the BHs in order to check
if they can be migrated. If __buffer_migrate_page() holds the lock at
the same time that ll_rw_block() wants it, the BH is skipped and I/O is
not issued for these blocks, and squashfs ends up seeing
!buffer_uptodate() and erroring out.
On newer kernels, squashfs doesn't use ll_rw_block() anymore, but I
still see other users of that function in other filesystems, and AFAICS
the underlying problem of the race with __buffer_migrate_page() has not
yet been fixed.
I'd be happy to receive any suggestions about the right way to fix this.
Thank you.
next reply other threads:[~2021-06-01 13:23 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-01 13:23 Vincent Whitchurch [this message]
2021-06-02 14:52 ` __buffer_migrate_page() vs ll_rw_block() Jan Kara
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=20210601132316.GA27976@axis.com \
--to=vincent.whitchurch@axis.com \
--cc=jack@suse.cz \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
/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).