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From:   David Howells <dhowells@redhat.com>
In-Reply-To: <Y/TbYGXC4HKunymf@casper.infradead.org>
References: <Y/TbYGXC4HKunymf@casper.infradead.org> <20230221184225.0e734f0e@canb.auug.org.au> <20230221174401.7198357d@canb.auug.org.au> <20230220152933.1ab8fa4a@canb.auug.org.au> <Y/N8hVWeR3AjssUC@casper.infradead.org> <20230220190157.3b43b9a7@canb.auug.org.au> <Y/Pe2xHklSr1hDtz@casper.infradead.org> <2351091.1676963957@warthog.procyon.org.uk> <2885897.1676990364@warthog.procyon.org.uk>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     dhowells@redhat.com, Stephen Rothwell <sfr@canb.auug.org.au>,
        "Vishal Moola (Oracle)" <vishal.moola@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Steve French <smfrench@gmail.com>,
        Steve French <stfrench@microsoft.com>,
        Shyam Prasad N <nspmangalore@gmail.com>,
        Rohith Surabattula <rohiths.msft@gmail.com>,
        Tom Talpey <tom@talpey.com>, Paulo Alcantara <pc@cjr.nz>,
        Jeff Layton <jlayton@kernel.org>, linux-cifs@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-next@vger.kernel.org
Subject: Re: linux-next: manual merge of the mm-stable tree with the cifs tree
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Date:   Tue, 21 Feb 2023 15:05:26 +0000
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Matthew Wilcox <willy@infradead.org> wrote:

> > +			start = folio_pos(folio); /* May regress with THPs */
> 
> What does this comment mean?

"start" may end up going backwards if it's pointing to the middle of a folio.

> > +			/* At this point we hold neither the i_pages lock nor the
> > +			 * page lock: the page may be truncated or invalidated
> > +			 * (changing page->mapping to NULL), or even swizzled
> > +			 * back from swapper_space to tmpfs file mapping
> 
> Where does this comment come from?  This is cifs, not tmpfs.  You'll
> never be asked to writeback a page from the swap cache.  Dirty pages
> can be truncated, so the first half of the comment is still accurate.
> I'd rather it moved down to below the folio lock, and was rephrased
> so it described why we're checking everything again.

I picked it up into afs from somewhere - nfs maybe?  The same comment is in
fs/btrfs/extent_io.c.  grep for 'swizzled' in fs/.  You modified the comment
in b93b016313b3ba8003c3b8bb71f569af91f19fc7 in 2018, so it's been around a
while.

David