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Date:   Wed, 9 Jan 2019 10:25:43 -0800
From:   Linus Torvalds <>
To:     Dave Chinner <>
Cc:     Jiri Kosina <>,
        Matthew Wilcox <>,
        Jann Horn <>,
        Andrew Morton <>,
        Greg KH <>,
        Peter Zijlstra <>,
        Michal Hocko <>, Linux-MM <>,
        kernel list <>,
        Linux API <>
Subject: Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged

On Tue, Jan 8, 2019 at 8:39 PM Dave Chinner <> 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.


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