linux-kernel - Re: [PATCH] mm: always respect QUEUE_FLAG_STABLE

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Date:   Thu, 4 May 2023 16:37:06 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Ilya Dryomov <idryomov@...il.com>
Cc:     Christoph Hellwig <hch@....de>, Jan Kara <jack@...e.cz>,
        Johannes Thumshirn <johannes.thumshirn@....com>,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: always respect QUEUE_FLAG_STABLE_WRITES on the block
 device

On Thu, May 04, 2023 at 05:07:10PM +0200, Ilya Dryomov wrote:
> On Thu, May 4, 2023 at 4:16 PM Matthew Wilcox <willy@...radead.org> wrote:
> > I hate both of these patches ;-)  What we should do is add
> > AS_STABLE_WRITES, have the appropriate places call
> > mapping_set_stable_writes() and then folio_wait_stable() becomes
> >
> >         if (mapping_test_stable_writes(folio->mapping))
> >                 folio_wait_writeback(folio);
> >
> > and we remove all the dereferences (mapping->host->i_sb->s_iflags, plus
> > whatever else is going on there)
> 
> Hi Matthew,
> 
> We would still need something resembling Christoph's suggestion for
> 5.10 and 5.15 (at least).  Since this fixes a regression, would you
> support merging the "ugly" version to facilitate backports or would
> you rather see the AS/mapping-based refactor first?

That's a terrible way of developing for Linux.  First, do it the right
way for mainline.  Then, see how easy the patch is to backport to relevant
kernel versions; if it's too ugly to cherry-pick, do something equivalent.
But never start out with the premise "This must be backported, so do it
as simply as possible first".