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Date: Sat, 20 Jan 2024 10:13:19 +0800
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Subject: Re: [PATCH v2] mm: memory: move mem_cgroup_charge() into
 alloc_anon_folio()
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To: Michal Hocko <mhocko@suse.com>
CC: Andrew Morton <akpm@linux-foundation.org>, <linux-mm@kvack.org>,
	<linux-kernel@vger.kernel.org>, <ryan.roberts@arm.com>, Matthew Wilcox
	<willy@infradead.org>, David Hildenbrand <david@redhat.com>
References: <20240117103954.2756050-1-wangkefeng.wang@huawei.com>
 <ZalK3suIskEyaR7m@tiehlicka>
 <c7b1cc8e-c434-4c86-972e-4a105524646c@huawei.com>
 <ZaosK59cRa27K9zW@tiehlicka>
 <14ae628d-a9ef-42f3-9201-e90c5c88c133@huawei.com>
 <ZaqZPxDUOuxRWB5l@tiehlicka>
From: Kefeng Wang <wangkefeng.wang@huawei.com>
In-Reply-To: <ZaqZPxDUOuxRWB5l@tiehlicka>
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On 2024/1/19 23:46, Michal Hocko wrote:
> On Fri 19-01-24 20:59:22, Kefeng Wang wrote:
>>>>> GFP_TRANSHUGE_LIGHT is more interesting though because those do not dive
>>>>> into the direct reclaim at all. With the current code they will reclaim
>>>>> charges to free up the space for the allocated THP page and that defeats
>>>>> the light mode. I have a vague recollection of preparing a patch to
>>>>
>>>> We are interesting to GFP_TRANSHUGE_LIGHT and _GFP_NORETRY as mentioned
>>>> above.
>>>
>>> if mTHP can be smaller than COSTLY_ORDER then you are correct and
>>> NORETRY makes a difference. Please mention that in the changelog as
>>> well.
>>>
>>
>> For memory cgroup charge, _GFP_NORETRY checked to make us directly skip
>> mem_cgroup_oom(), it has no concern with folio order or COSTLY_ORDER when
>> check _GFP_NORETRY in try_charge_memcg(), so I think NORETRY should
>> always make difference for all large order folio.
> 
> we do not OOM on COSTLY_ORDER (see mem_cgroup_oom). So NORETRY really
> makes a difference for small orders.

I see what you mean, but we may describe the different processes, if
GFP_TRANSHUGE | __GFP_NORETRY returned from vma_thp_gfp_mask(),
then we never involved with mem_cgroup_oom(), since mem_cgroup_oom()
will be skipped in try_charge_memcg(), that is what I want to say,
and in this case, no oom for order < COSTLY_ORDER or order > 
COSTLY_ORDER. But if GFP is GFP_TRANHUGE, then we may enter 
mem_cgroup_oom(), and maybe oom if order < COSTLY_ORDER.

So Yes, NORETRY really makes a difference for small orders.