Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged

[Linus Torvalds]

On Tue, Jan 8, 2019 at 8:39 PM Dave Chinner <david@xxxxxxxxxxxxx> 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