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Date:   Sun, 6 Mar 2022 10:27:09 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Hugh Dickins <hughd@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mikulas Patocka <mpatocka@redhat.com>,
        Zdenek Kabelac <zkabelac@redhat.com>,
        Lukas Czerner <lczerner@redhat.com>,
        "Darrick J. Wong" <djwong@kernel.org>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Christoph Hellwig <hch@lst.de>, Borislav Petkov <bp@suse.de>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH mmotm] tmpfs: do not allocate pages on read
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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.