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
