Re: [RFC v3 6/7] shm: wait for pins to be released when sealing

[David Herrmann <dh.herrmann@gmail.com>]

Hi

On Wed, Jul 16, 2014 at 12:09 PM, Hugh Dickins <hughd@google.com> wrote:
> On Fri, 13 Jun 2014, David Herrmann wrote:
>
>> We currently fail setting SEAL_WRITE in case there're pending page
>> references. This patch extends the pin-tests to wait up to 150ms for all
>> references to be dropped. This is still not perfect in that it doesn't
>> account for harmless read-only pins, but it's much better than a hard
>> failure.
>>
>> Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
>
> Right, I didn't look through the patch itself, just compared the result
> with what I sent.  Okay, you prefer to separate out shmem_tag_pins().

The main reason why I split both is to avoid goto-label "restart" and
"restart2".

> Yes, it looks fine.  There's just one change I'd like at this stage,
> something I realized shortly after sending the code fragment: please
> add a call to lru_add_drain() at the head of shmem_tag_pins().  The
> reason being that lru_add_drain() is local to the cpu, so cheap, and
> in many cases will bring down all the raised refcounts right then.
>
> Whereas lru_add_drain_all() in the first scan of shmem_wait_for_pins()
> is much more expensive, involving inter-processor interrupts to do
> that on all cpus: it is appropriate to call it at that point, but we
> really ought to try the cheaper lru_add_drain() at the earlier stage.

I added an lru_add_drain_all() to my shmem_test_pins() function in
Patch 2/7. This patch dropped it again as your wait_for_pins() already
included it and it's quite expensive. But yes, the local
lru_add_drain() makes perfect sense. Fixed!

Thanks
David

> I would also like never to embark on this scan of the radix_tree
> and wait for pins, if the pages were never given out in a VM_SHARED
> mapping - or is that unrealistic, because every memfd is read-write,
> and typical initialization expected to be by mmap() rather than write()?
> But anyway, you're quite right not to get into that at this stage:
> it's best left as an optimization once the basics are safely in.

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