ceph-devel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Jeff Layton <jlayton@kernel.org>
To: Jan Kara <jack@suse.cz>
Cc: Ilya Dryomov <idryomov@gmail.com>, ceph-devel@vger.kernel.org
Subject: Re: Hole punch races in Ceph
Date: Thu, 22 Apr 2021 07:43:16 -0400	[thread overview]
Message-ID: <fca235a3e2f0fe2a96d0482c3586372a9580c98b.camel@kernel.org> (raw)
In-Reply-To: <20210422111557.GE26221@quack2.suse.cz>

On Thu, 2021-04-22 at 13:15 +0200, Jan Kara wrote:
> Hello,
> 
> I'm looking into how Ceph protects against races between page fault and
> hole punching (I'm unifying protection for this kind of races among
> filesystems) and AFAICT it does not. What I have in mind in particular is a
> race like:
> 
> CPU1					CPU2
> 
> ceph_fallocate()
>   ...
>   ceph_zero_pagecache_range()
> 					ceph_filemap_fault()
> 					  faults in page in the range being
> 					  punched
>   ceph_zero_objects()
> 
> And now we have a page in punched range with invalid data. If
> ceph_page_mkwrite() manages to squeeze in at the right moment, we might
> even associate invalid metadata with the page I'd assume (but I'm not sure
> whether this would be harmful). Am I missing something?
> 
> 								Honza

No, I don't think you're missing anything. If ceph_page_mkwrite happens
to get called at an inopportune time then we'd probably end up writing
that page back into the punched range too. What would be the best way to
fix this, do you think?

One idea:

We could lock the pages we're planning to punch out first, then
zero/punch out the objects on the OSDs, and then do the hole punch in
the pagecache? Would that be sufficient to close the race?

-- 
Jeff Layton <jlayton@kernel.org>


  reply	other threads:[~2021-04-22 11:43 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-22 11:15 Jan Kara
2021-04-22 11:43 ` Jeff Layton [this message]
2021-04-22 12:02   ` Jan Kara
2021-04-22 12:05     ` Jeff Layton

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=fca235a3e2f0fe2a96d0482c3586372a9580c98b.camel@kernel.org \
    --to=jlayton@kernel.org \
    --cc=ceph-devel@vger.kernel.org \
    --cc=idryomov@gmail.com \
    --cc=jack@suse.cz \
    --subject='Re: Hole punch races in Ceph' \
    /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).