linux-kernel - [LSF/MM TOPIC] do we really need PG

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Message-ID: <1488120164.2948.4.camel@redhat.com>
Date:   Sun, 26 Feb 2017 09:42:44 -0500
From:   Jeff Layton <jlayton@...hat.com>
To:     linux-mm <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     lsf-pc <lsf-pc@...ts.linuxfoundation.org>,
        Neil Brown <neilb@...e.de>
Subject: [LSF/MM TOPIC] do we really need PG_error at all?

Proposing this as a LSF/MM TOPIC, but it may turn out to be me just not
understanding the semantics here.

As I was looking into -ENOSPC handling in cephfs, I noticed that
PG_error is only ever tested in one place [1] __filemap_fdatawait_range,
which does this:

	if (TestClearPageError(page))
		ret = -EIO;

This error code will override any AS_* error that was set in the
mapping. Which makes me wonder...why don't we just set this error in the
mapping and not bother with a per-page flag? Could we potentially free
up a page flag by eliminating this?

The main argument I could see for keeping it is that removing it might
subtly change the behavior of sync_file_range if you have tasks syncing
different ranges in a file concurrently. I'm not sure if that would
break any guarantees though.

Even if we do need it, I think we might need some cleanup here anyway. A
lot of readpage operations end up setting that flag when they hit an
error. Isn't it wrong to return an error on fsync, just because we had a
read error somewhere in the file in a range that was never dirtied?

--
[1]: there is another place in f2fs, but it's more or less equivalent to
the call site in __filemap_fdatawait_range.

-- 
Jeff Layton <jlayton@...hat.com>