Re: [PATCH 2/2] mm: memcontrol: drop unnecessary lru locking from mem_cgroup_migrate()

[Johannes Weiner <jweiner@fb.com>]

> On Feb 7, 2016, at 1:41 PM, Vladimir Davydov <vdavydov@virtuozzo.com> wrote:
> 
>> On Thu, Feb 04, 2016 at 03:07:47PM -0500, Johannes Weiner wrote:
>> Migration accounting in the memory controller used to have to handle
>> both oldpage and newpage being on the LRU already; fuse's page cache
>> replacement used to pass a recycled newpage that had been uncharged
>> but not freed and removed from the LRU, and the memcg migration code
>> used to uncharge oldpage to "pass on" the existing charge to newpage.
>> 
>> Nowadays, pages are no longer uncharged when truncated from the page
>> cache, but rather only at free time, so if a LRU page is recycled in
>> page cache replacement it'll also still be charged. And we bail out of
>> the charge transfer altogether in that case. Tell commit_charge() that
>> we know newpage is not on the LRU, to avoid taking the zone->lru_lock
>> unnecessarily from the migration path.
>> 
>> But also, oldpage is no longer uncharged inside migration. We only use
>> oldpage for its page->mem_cgroup and page size, so we don't care about
>> its LRU state anymore either. Remove any mention from the kernel doc.
>> 
>> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
>> Suggested-by: Hugh Dickins <hughd@google.com>
> 
> Acked-by: Vladimir Davydov <vdavydov@virtuozzo.com>

Thanks!

> @@ -5483,6 +5483,7 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage)
>    unsigned int nr_pages;
>    bool compound;
> 
> +    VM_BUG_ON_PAGE(PageLRU(newpage), newpage);

That's actually possible for fuse. But in that case newpage is charged and we bail.
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