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Subject: Re: [PATCH v1 08/12] powerpc/pseries: CMM: Implement balloon
 compaction
To: Michael Ellerman <mpe@ellerman.id.au>, linux-kernel@vger.kernel.org
References: <20191031142933.10779-1-david@redhat.com>
 <20191031142933.10779-9-david@redhat.com>
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From: David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
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Date: Tue, 12 Nov 2019 12:12:33 +0100
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

On 12.11.19 11:46, Michael Ellerman wrote:
> David Hildenbrand <david@redhat.com> writes:
>> On 31.10.19 15:29, David Hildenbrand wrote:
>>> We can now get rid of the cmm_lock and completely rely on the balloon
>>> compaction internals, which now also manage the page list and the lock.
> ...
>>> +
>>> +static int cmm_migratepage(struct balloon_dev_info *b_dev_info,
>>> +=09=09=09   struct page *newpage, struct page *page,
>>> +=09=09=09   enum migrate_mode mode)
>>> +{
>>> +=09unsigned long flags;
>>> +
>>> +=09/*
>>> +=09 * loan/"inflate" the newpage first.
>>> +=09 *
>>> +=09 * We might race against the cmm_thread who might discover after ou=
r
>>> +=09 * loan request that another page is to be unloaned. However, once
>>> +=09 * the cmm_thread runs again later, this error will automatically
>>> +=09 * be corrected.
>>> +=09 */
>>> +=09if (plpar_page_set_loaned(newpage)) {
>>> +=09=09/* Unlikely, but possible. Tell the caller not to retry now. */
>>> +=09=09pr_err_ratelimited("%s: Cannot set page to loaned.", __func__);
>>> +=09=09return -EBUSY;
>>> +=09}
>>> +
>>> +=09/* balloon page list reference */
>>> +=09get_page(newpage);
>>> +
>>> +=09spin_lock_irqsave(&b_dev_info->pages_lock, flags);
>>> +=09balloon_page_insert(b_dev_info, newpage);
>>> +=09balloon_page_delete(page);
>>
>> I think I am missing a b_dev_info->isolated_pages-- here.
>=20
> I don't know this code at all, but looking at other balloon drivers they
> do seem to do that in roughly the same spot.
>=20
> I'll add it, how can we test that it's correct?

It's certainly correct. We increment when we isolate=20
(balloon_page_isolate()) and decrement when we un-isolate.

Un-isolate happens when we putback a isolated page=20
(balloon_page_putback() - migration aborted) or when we successfully=20
migrate it (via balloon_page_migrate()).

The issue is that we cannot decrement in balloon_page_migrate(), as we=20
have to hold the b_dev_info->pages_lock. That's why we have to do it in=20
the registered callback under lock.

Please note that b_dev_info->isolated_pages is only needed for a sanity=20
check in balloon_page_dequeue(). That's why I didn't notice during=20
testing. I wonder if we should at some point rip out that sanity check ...

Thanks and cheers!

>=20
> cheers
>=20


--=20

Thanks,

David / dhildenb