linux-kernel - Obsolete comment on page swizzling (written by Hugh)?

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Message-ID: <2889872.1676993446@warthog.procyon.org.uk>
Date:   Tue, 21 Feb 2023 15:30:46 +0000
From:   David Howells <dhowells@...hat.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     dhowells@...hat.com, Stephen Rothwell <sfr@...b.auug.org.au>,
        "Vishal Moola (Oracle)" <vishal.moola@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steve French <smfrench@...il.com>,
        Steve French <stfrench@...rosoft.com>,
        Shyam Prasad N <nspmangalore@...il.com>,
        Rohith Surabattula <rohiths.msft@...il.com>,
        Tom Talpey <tom@...pey.com>, Paulo Alcantara <pc@....nz>,
        Jeff Layton <jlayton@...nel.org>, linux-cifs@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-next@...r.kernel.org
Subject: Obsolete comment on page swizzling (written by Hugh)?

David Howells <dhowells@...hat.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@...eo.com>
	Date:   Fri Nov 15 18:52:38 2002 -0800

	[PATCH] handle pages which alter their ->mapping

	Patch from Hugh Dickins <hugh@...itas.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