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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 16 Jan 2019 17:49:46 +1200
Message-ID: <CAHk-=wje=2Pndo+xZ5fLJ9VCoo6NYLV_a9D8mxpuSTFdz3eGMg@mail.gmail.com>
Subject: Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged
To:     Dave Chinner <david@fromorbit.com>
Cc:     Dominique Martinet <asmadeus@codewreck.org>,
        Jiri Kosina <jikos@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Jann Horn <jannh@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Michal Hocko <mhocko@suse.com>, Linux-MM <linux-mm@kvack.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>
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On Wed, Jan 16, 2019 at 4:54 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Wed, Jan 16, 2019 at 11:45 AM Dave Chinner <david@fromorbit.com> wrote:
> >
> > I'm assuming that you can invalidate the page cache reliably by a
> > means that does not repeated require probing to detect invalidation
> > has occurred. I've mentioned one method in this discussion
> > already...
>
> Yes. And it was made clear to you that it was a bug in xfs dio and
> what the right thing to do was.

Side note: I actually think we *do* the right thing. Even for xfs. I
couldn't find the alleged place that invalidates the page cache on dio
reads.

The *generic* dio code only does it for writes (which is correct and
fine). And maybe xfs has some extra invalidation, but I don't see it.

So I actually hope your "you can use direct-io read to do directed
invalidating of the page cache" isn't true. I admittedly did *not* try
to delve very deeply into it, but the invalidates I found looked
correct. The generic code does it for writes, and at least ext4 does
the "writeback and wait" for reads.

There *does* seem to be a 'invalidate_inode_pages2_range()' call in
iomap_dio_rw(). That has a *comment* that says it only is for writes,
but it looks to me like it would trigger for reads too.

Just a plain bug/oversight? Or me misreading things.

So yes, maybe xfs does that "invalidate on read", but it really seems
to be just a bug. If the xfs people insist on keeping the bug, fine
(looks like gfs2 and xfs are the only users), but it seems kind of
sad.

             Linus