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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 27 Nov 2019 09:29:44 -0800
Message-ID: <CAHk-=wh5yeAj47x5EAuec7NCgFyYV1VtB9A+nBq+bVP9HmeepQ@mail.gmail.com>
Subject: Re: [PATCH] mm/filemap: do not allocate cache pages beyond end of
 file at read
To:     Steven Whitehouse <swhiteho@redhat.com>
Cc:     =?UTF-8?Q?Andreas_Gr=C3=BCnbacher?= <andreas.gruenbacher@gmail.com>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Linux-MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Johannes Weiner <hannes@cmpxchg.org>,
        "cluster-devel@redhat.com" <cluster-devel@redhat.com>,
        Ronnie Sahlberg <lsahlber@redhat.com>,
        Steve French <sfrench@samba.org>,
        Andreas Gruenbacher <agruenba@redhat.com>,
        Bob Peterson <rpeterso@redhat.com>
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On Wed, Nov 27, 2019 at 7:42 AM Steven Whitehouse <swhiteho@redhat.com> wrote:
>
> I'll leave the finer details to Andreas here, since it is his patch, and
> hopefully we can figure out a good path forward.

As mentioned, I don't _hate_ that patch (ok, I seem to have typoed it
and said that I don't "gate" it ;), so if that's what you guys really
want to do, I'm ok with it. But..

I do think you already get the data with the current case, from the
"short read" thing. So just changing the current generic read function
to check against the size first:

  --- a/mm/filemap.c
  +++ b/mm/filemap.c
  @@ -2021,9 +2021,9 @@ static ssize_t
generic_file_buffered_read(struct kiocb *iocb,
        unsigned int prev_offset;
        int error = 0;

  -     if (unlikely(*ppos >= inode->i_sb->s_maxbytes))
  +     if (unlikely(*ppos >= inode->i_size))
                return 0;
  -     iov_iter_truncate(iter, inode->i_sb->s_maxbytes);
  +     iov_iter_truncate(iter, inode->i_size);

        index = *ppos >> PAGE_SHIFT;
        prev_index = ra->prev_pos >> PAGE_SHIFT;

and you're done. Nice and clean.

Then in gfs2 you just notice the short read, and check at that point.
Sure, you'll also cut read-ahead to the old size boundary, but does
anybody _seriously_ believe that read-ahead matters when you hit the
"some other node write more data, we're reading past the old end"
case? I don't think that's the case.

But I _can_ live with the patch that adds the extra "cached only" bit.
It just honestly feels pointless.

               Linus