Subject: Obsolete comment on page swizzling (written by Hugh)?

From: David Howells <[email-masked]>
Date: Tue, 21 Feb 2023 15:30:46 +0000

David Howells <[email-masked]> 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 <[email-masked]>
	Date:   Fri Nov 15 18:52:38 2002 -0800

	[PATCH] handle pages which alter their ->mapping

	Patch from Hugh Dickins <[email-masked]>

	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