linux-ext4 - Re: [PATCH 08/13] iomap: assign current->backing_dev_info in iomap_file_buffered

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Date:   Tue, 23 May 2023 04:30:51 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Xiubo Li <xiubli@...hat.com>,
        Ilya Dryomov <idryomov@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Theodore Ts'o <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>,
        Damien Le Moal <dlemoal@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-block@...r.kernel.org, ceph-devel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        "open list:F2FS FILE SYSTEM" <linux-f2fs-devel@...ts.sourceforge.net>,
        cluster-devel@...hat.com, linux-xfs@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-mm@...ck.org,
        NeilBrown <neilb@...e.de>
Subject: Re: [PATCH 08/13] iomap: assign current->backing_dev_info in
 iomap_file_buffered_write

On Mon, May 22, 2023 at 06:06:27PM -0700, Darrick J. Wong wrote:
> On Fri, May 19, 2023 at 11:35:16AM +0200, Christoph Hellwig wrote:
> > Move the assignment to current->backing_dev_info from the callers into
> > iomap_file_buffered_write to reduce boiler plate code and reduce the
> > scope to just around the page dirtying loop.
> > 
> > Note that zonefs was missing this assignment before.
> 
> I'm still wondering (a) what the hell current->backing_dev_info is for,
> and (b) if we need it around the iomap_unshare operation.
> 
> $ git grep current..backing_dev_info
[results show it only set, never used]
> 
> AFAICT nobody uses it at all?  Unless there's some bizarre user that
> isn't extracting it from @current?
> 
> Oh, hey, new question (c) isn't this set incorrectly for xfs realtime
> files?

Some git archaelogy ...

This was first introduced in commit 2f45a06517a62 (in the
linux-fullhistory tree) in 2002 by one Andrew Morton.  At the time,
it added this check to the page scanner:

+                               if (page->pte.direct ||
+                                       page->mapping->backing_dev_info ==
+                                               current->backing_dev_info) {
+                                       wait_on_page_writeback(page);
+                               }

AFAICT (the code went through some metamorphoses in the intervening
twenty years), the last use of it ended up in current_may_throttle(),
and it was removed in March 2022 by Neil Brown in commit b9b1335e6403.
Since then, there have been no users of task->backing_dev_info, and I'm
pretty sure it can go away.