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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ > 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@xxxxxxxxxx/ > > 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@xxxxxxxxxx> > Reported-by: Lukas Czerner <lczerner@xxxxxxxxxx> > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> 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@xxxxxx>
