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

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Message-ID: <0bea2b1c-ddb1-f2bf-8ef7-b83d6a6404fc@gmail.com>
Date:   Tue, 28 Feb 2017 12:12:14 +0200
From:   Boaz Harrosh <openosd@...il.com>
To:     Jeff Layton <jlayton@...hat.com>, NeilBrown <neilb@...e.com>,
        Andreas Dilger <adilger@...ger.ca>
Cc:     linux-block@...r.kernel.org,
        linux-scsi <linux-scsi@...r.kernel.org>,
        lsf-pc <lsf-pc@...ts.linuxfoundation.org>,
        Neil Brown <neilb@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        linux-mm <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [Lsf-pc] [LSF/MM TOPIC] do we really need PG_error at all?

On 02/28/2017 03:11 AM, Jeff Layton wrote:
<>
> 
> I'll probably have questions about the read side as well, but for now it
> looks like it's mostly used in an ad-hoc way to communicate errors
> across subsystems (block to fs layer, for instance).

If memory does not fail me it used to be checked long time ago in the
read-ahead case. On the buffered read case, the first page is read synchronous
and any error is returned to the caller, but then a read-ahead chunk is
read async all the while the original thread returned to the application.
So any errors are only recorded on the page-bit, since otherwise the uptodate
is off and the IO will be retransmitted. Then the move to read_iter changed
all that I think.
But again this is like 5-6 years ago, and maybe I didn't even understand
very well.

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

I would like a Documentation of all this as well please. Where are the
tests for this?

Thanks
Boaz