From mboxrd@z Thu Jan  1 00:00:00 1970
From: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged
Date: Wed, 9 Jan 2019 10:25:43 -0800
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To: Dave Chinner <david@fromorbit.com>
Cc: 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 Tue, Jan 8, 2019 at 8:39 PM Dave Chinner <david@fromorbit.com> wrote:
>
> FWIW, I just realised that the easiest, most reliable way to
> invalidate the page cache over a file range is simply to do a
> O_DIRECT read on it.

If that's the case, that's actually an O_DIRECT bug.

It should only invalidate the caches on write.

On reads, it wants to either _flush_ any direct caches before the
read, or just take the data from the caches. At no point is
"invalidate" a valid model.

Of course, I'm not in the least bit shocked if O_DIRECT is buggy like
this. But looking at least at the ext4 routine, the read just does

        ret = filemap_write_and_wait_range(mapping, iocb->ki_pos,

and I don't see any invalidation.

Having read access to a file absolutely should *not* mean that you can
flush caches on it. That's a write op.

Any filesystem that invalidates the caches on read is utterly buggy.

Can you actually point to such a thing? Let's get that fixed, because
it's completely wrong regardless of this whole mincore issue.

               Linus