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

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Date:   Tue, 28 Feb 2017 10:32:34 +1100
From:   NeilBrown <neilb@...e.com>
To:     Andreas Dilger <adilger@...ger.ca>,
        Jeff Layton <jlayton@...hat.com>
Cc:     James Bottomley <James.Bottomley@...senPartnership.com>,
        linux-mm <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        lsf-pc <lsf-pc@...ts.linuxfoundation.org>,
        Neil Brown <neilb@...e.de>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        linux-block@...r.kernel.org
Subject: Re: [LSF/MM TOPIC] do we really need PG_error at all?

On Mon, Feb 27 2017, Andreas Dilger wrote:

>
> My thought is that PG_error is definitely useful for applications to get
> correct errors back when doing write()/sync_file_range() so that they know
> there is an error in the data that _they_ wrote, rather than receiving an
> error for data that may have been written by another thread, and in turn
> clearing the error from another thread so it *doesn't* know it had a write
> error.

It might be useful in that way, but it is not currently used that way.
Such usage would be a change in visible behaviour.

sync_file_range() calls filemap_fdatawait_range(), which calls
filemap_check_errors().
If there have been any errors in the file recently, inside or outside
the range, the latter will return an error which will propagate up.

>
> As for stray sync() clearing PG_error from underneath an application, that
> shouldn't happen since filemap_fdatawait_keep_errors() doesn't clear errors
> and is used by device flushing code (fdatawait_one_bdev(), wait_sb_inodes()).

filemap_fdatawait_keep_errors() calls __filemap_fdatawait_range() which
clears PG_error on every page.
What it doesn't do is call filemap_check_errors(), and so doesn't clear
AS_ENOSPC or AS_EIO.

NeilBrown

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