Re: [PATCH mmotm v2] tmpfs: do not allocate pages on read

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Mar 07, 2022 at 07:44:34AM +0100, Christoph Hellwig wrote:
> On Sun, Mar 06, 2022 at 02:59:05PM -0800, Hugh Dickins wrote:
> > Mikulas asked in
> > https://lore.kernel.org/linux-mm/alpine.LRH.2.02.2007210510230.6959@file01.intranet.prod.int.rdu2.redhat.com/
> > Do we still need a0ee5ec520ed ("tmpfs: allocate on read when stacked")?
> > 
> > Lukas noticed this unusual behavior of loop device backed by tmpfs in
> > https://lore.kernel.org/linux-mm/20211126075100.gd64odg2bcptiqeb@work/
> > 
> > Normally, shmem_file_read_iter() copies the ZERO_PAGE when reading holes;
> > but if it looks like it might be a read for "a stacking filesystem", it
> > allocates actual pages to the page cache, and even marks them as dirty.
> > And reads from the loop device do satisfy the test that is used.
> > 
> > This oddity was added for an old version of unionfs, to help to limit
> > its usage to the limited size of the tmpfs mount involved; but about
> > the same time as the tmpfs mod went in (2.6.25), unionfs was reworked
> > to proceed differently; and the mod kept just in case others needed it.
> > 
> > Do we still need it? I cannot answer with more certainty than "Probably
> > not". It's nasty enough that we really should try to delete it; but if
> > a regression is reported somewhere, then we might have to revert later.
> > 
> > It's not quite as simple as just removing the test (as Mikulas did):
> > xfstests generic/013 hung because splice from tmpfs failed on page not
> > up-to-date and page mapping unset.  That can be fixed just by marking
> > the ZERO_PAGE as Uptodate, which of course it is: do so in
> > pagecache_init() - it might be useful to others than tmpfs.
> > 
> > My intention, though, was to stop using the ZERO_PAGE here altogether:
> > surely iov_iter_zero() is better for this case?  Sadly not: it relies
> > on clear_user(), and the x86 clear_user() is slower than its copy_user():
> > https://lore.kernel.org/lkml/2f5ca5e4-e250-a41c-11fb-a7f4ebc7e1c9@google.com/
> > 
> > But while we are still using the ZERO_PAGE, let's stop dirtying its
> > struct page cacheline with unnecessary get_page() and put_page().
> > 
> > Reported-by: Mikulas Patocka <mpatocka@redhat.com>
> > Reported-by: Lukas Czerner <lczerner@redhat.com>
> > Signed-off-by: Hugh Dickins <hughd@google.com>
> 
> I would have split the uptodate setting of ZERO_PAGE into a separate,
> clearly documented patch, but otherwise this looks good:
> 
> Reviewed-by: Christoph Hellwig <hch@lst.de>

I've long wondered (for my own nefarious purposes) why tmpfs files
didn't just grab the zero page, so:

Acked-by: Darrick J. Wong <djwong@kernel.org>

--D