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From:   David Howells <dhowells@redhat.com>
In-Reply-To: <2888446.1676992843@warthog.procyon.org.uk>
References: <2888446.1676992843@warthog.procyon.org.uk> <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>
cc:     Matthew Wilcox <willy@infradead.org>,
        Hugh Dickins <hughd@google.com>
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: Obsolete comment on page swizzling (written by Hugh)?
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Date:   Tue, 21 Feb 2023 15:30:46 +0000
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David Howells <dhowells@redhat.com> wrote:

> > > +			/* 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.
> 
> Actually, it's in v6.2 cifs and I just move it in the patch where I copy the
> afs writepages implementation into cifs.  afs got it in 2007 when I added
> write support[1] and I suspect I copied it from cifs.  cifs got it in 2005
> when Steve added writepages support[2].  I think he must've got it from
> fs/mpage.c and the comment there is prehistoric.

The ultimate source is Hugh Dickins, it would seem:

	commit 820ef9df32856bb54fe5bc995153feb276420e15
	Author: Andrew Morton <akpm@digeo.com>
	Date:   Fri Nov 15 18:52:38 2002 -0800

	[PATCH] handle pages which alter their ->mapping

	Patch from Hugh Dickins <hugh@veritas.com>

	tmpfs failed fsx+swapout tests after many hours, a page found zeroed.
	Not a truncate problem, but mirror image of earlier truncate problems:
	swap goes through mpage_writepages, which must therefore allow for a
	sudden swizzle back to file identity.

	Second time this caught us, so I've audited the tree for other places
	which might be surprised by such swizzling.  The only others I found
	were (perhaps) in the parisc and sparc64 flush_dcache_page called
	from do_generic_mapping_read on a looped tmpfs file which is also
	mmapped; but that's a very marginal case, I wanted to understand it
	better before making any edit, and now realize that hch's sendfile
	in loop eliminates it (now go through do_shmem_file_read instead:
	similar but crucially this locks the page when raising its count,
	which is enough to keep vmscan from interfering).

Maybe we should delete or amend the comment now?

David