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    SubjectRe: [PATCH mmotm] tmpfs: do not allocate pages on read
    On Fri, Mar 04, 2022 at 09:09:01PM -0800, Hugh Dickins wrote:
    > 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; doing so here in
    > shmem_file_read_iter() is distasteful, but seems to be the best way.

    Shouldn't we set ZERO_PAGE uptodate during early init code as it, uh,
    is per definition uptodate all the time?

    >
    > 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/

    Oh, that's sad as just using clear_user would be the right thing to
    here.

    > 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>

    But except for maybe making sure that ZERO_PAGE is always marked
    uptodate this does looks good to me.

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