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From: Al Viro <viro@ZenIV.linux.org.uk>
To: Miklos Szeredi <mszeredi@redhat.com>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	Jens Axboe <axboe@kernel.dk>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 06/11] pipe: no need to confirm page cache buf
Date: Tue, 27 Sep 2016 04:40:35 +0100	[thread overview]
Message-ID: <20160927034035.GG19539@ZenIV.linux.org.uk> (raw)
In-Reply-To: <1473842236-28655-7-git-send-email-mszeredi@redhat.com>

On Wed, Sep 14, 2016 at 10:37:11AM +0200, Miklos Szeredi wrote:

> Things could happen to that page that make it not uptodate while sitting in
> the pipe, but it's questionable whether we should care about that.
> Checking for being uptodate in the face of such page state change is always
> going to be racy.

I'm not sure it's the right thing to do here; that area looks like a victim
of serious bitrot - once upon a time it was ->map(), which used to lock
page, verity that it's valid, and kmap it.  ->unmap() did kunmap + unlock.

Then the validate part got split off (->pin(), later renamed to ->confirm()),
with lock _not_ held over the kmap/kunmap.  That's the point when it got racy,
AFAICS.  OTOH, I would really hate to hold a page locked over e.g. copying to
userland - too easy to get deadlocks that way.

Jens, could you comment?  Pages definitely shouldn't be getting into pipe
without having been uptodate; the question is what (if anything) should be
done about having a page go non-uptodate (on truncate, hole-punching, etc.)
while a reference to it is sitting in a pipe buffer.

  reply	other threads:[~2016-09-27  3:40 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-09-14  8:37 [PATCH 00/11] splice cleanups Miklos Szeredi
2016-09-14  8:37 ` [PATCH 01/11] pipe: add pipe_buf_get() helper Miklos Szeredi
2016-09-14  8:37 ` [PATCH 02/11] pipe: add pipe_buf_release() helper Miklos Szeredi
2016-09-14  8:37 ` [PATCH 03/11] pipe: add pipe_buf_confirm() helper Miklos Szeredi
2016-09-14  8:37 ` [PATCH 04/11] pipe: add pipe_buf_steal() helper Miklos Szeredi
2016-09-14  8:37 ` [PATCH 05/11] pipe: fix comment in pipe_buf_operations Miklos Szeredi
2016-09-14  8:37 ` [PATCH 06/11] pipe: no need to confirm page cache buf Miklos Szeredi
2016-09-27  3:40   ` Al Viro [this message]
2016-09-27  7:34     ` Miklos Szeredi
2016-09-14  8:37 ` [PATCH 07/11] pipe: remove generic_pipe_buf_confirm() Miklos Szeredi
2016-09-16 11:23   ` Christoph Hellwig
2016-09-14  8:37 ` [PATCH 08/11] filemap: add get_page_for_read() helper Miklos Szeredi
2016-09-27  3:43   ` Al Viro
2016-09-14  8:37 ` [PATCH 09/11] splice: use get_page_for_read() Miklos Szeredi
2016-09-27  3:45   ` Al Viro
2016-09-14  8:37 ` [PATCH 10/11] splice: don't check i_size in generic_file_splice_read() Miklos Szeredi
2016-09-14  8:37 ` [PATCH 11/11] splice: fold __generic_file_splice_read() into caller Miklos Szeredi
2016-09-14  8:55 ` [PATCH 00/11] splice cleanups Cedric Blancher
2016-09-14  9:30   ` Miklos Szeredi
2016-09-16 11:24 ` Christoph Hellwig
2016-09-27  3:55 ` Al Viro

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