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SubjectObsolete comment on page swizzling (written by Hugh)?
Date
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

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