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SubjectRe: Obsolete comment on page swizzling (written by Hugh)?
On Tue, 21 Feb 2023, David Howells wrote:
> 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?

Yes, that comment does not belong in afs or btrfs or cifs - though it
does explain why we have sometimes chosen to compare folio_mapping(folio)
with expected mapping, rather than against NULL.

But "now" is not the moment to amend it: it looks like these sources
are in flux at present. And truncate_cleanup_folio() has a "swizzles"
comment without even a mapping to compare with.

Hugh

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