linux-kernel - Re: [PATCH v2 2/4] mm: introduce cma_alloc

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Date:   Wed, 2 Dec 2020 17:00:37 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, hyesoo.yu@...sung.com,
        willy@...radead.org, iamjoonsoo.kim@....com, vbabka@...e.cz,
        surenb@...gle.com, pullip.cho@...sung.com, joaodias@...gle.com,
        hridya@...gle.com, sumit.semwal@...aro.org, john.stultz@...aro.org,
        Brian.Starkey@....com, linux-media@...r.kernel.org,
        devicetree@...r.kernel.org, robh@...nel.org,
        christian.koenig@....com, linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH v2 2/4] mm: introduce cma_alloc_bulk API

On 02.12.20 16:49, Michal Hocko wrote:
> On Wed 02-12-20 10:14:41, David Hildenbrand wrote:
>> On 01.12.20 18:51, Minchan Kim wrote:
>>> There is a need for special HW to require bulk allocation of
>>> high-order pages. For example, 4800 * order-4 pages, which
>>> would be minimum, sometimes, it requires more.
>>>
>>> To meet the requirement, a option reserves 300M CMA area and
>>> requests the whole 300M contiguous memory. However, it doesn't
>>> work if even one of those pages in the range is long-term pinned
>>> directly or indirectly. The other option is to ask higher-order
>>
>> My latest knowledge is that pages in the CMA area are never long term
>> pinned.
>>
>> https://lore.kernel.org/lkml/20201123090129.GD27488@dhcp22.suse.cz/
>>
>> "gup already tries to deal with long term pins on CMA regions and migrate
>> to a non CMA region. Have a look at __gup_longterm_locked."
>>
>> We should rather identify ways how that is still possible and get rid of
>> them.
>>
>>
>> Now, short-term pinnings and PCP are other issues where
>> alloc_contig_range() could be improved (e.g., in contrast to a FAST
>> mode, a HARD mode which temporarily disables the PCP, ...).
> 
> Agreed!
> 
>>> size (e.g., 2M) than requested order(64K) repeatedly until driver
>>> could gather necessary amount of memory. Basically, this approach
>>> makes the allocation very slow due to cma_alloc's function
>>> slowness and it could be stuck on one of the pageblocks if it
>>> encounters unmigratable page.
>>>
>>> To solve the issue, this patch introduces cma_alloc_bulk.
>>>
>>> 	int cma_alloc_bulk(struct cma *cma, unsigned int align,
>>> 		bool fast, unsigned int order, size_t nr_requests,
>>> 		struct page **page_array, size_t *nr_allocated);
>>>
>>> Most parameters are same with cma_alloc but it additionally passes
>>> vector array to store allocated memory. What's different with cma_alloc
>>> is it will skip pageblocks without waiting/stopping if it has unmovable
>>> page so that API continues to scan other pageblocks to find requested
>>> order page.
>>>
>>> cma_alloc_bulk is best effort approach in that it skips some pageblocks
>>> if they have unmovable pages unlike cma_alloc. It doesn't need to be
>>> perfect from the beginning at the cost of performance. Thus, the API
>>> takes "bool fast parameter" which is propagated into alloc_contig_range to
>>> avoid significat overhead functions to inrecase CMA allocation success
>>> ratio(e.g., migration retrial, PCP, LRU draining per pageblock)
>>> at the cost of less allocation success ratio. If the caller couldn't
>>> allocate enough, they could call it with "false" to increase success ratio
>>> if they are okay to expense the overhead for the success ratio.
>>
>> Just so I understand what the idea is:
>>
>> alloc_contig_range() sometimes fails on CMA regions when trying to
>> allocate big chunks (e.g., 300M). Instead of tackling that issue, you
>> rather allocate plenty of small chunks, and make these small allocations
>> fail faster/ make the allocations less reliable. Correct?
>>
>> I don't really have a strong opinion on that. Giving up fast rather than
>> trying for longer sounds like a useful thing to have - but I wonder if
>> it's strictly necessary for the use case you describe.
>>
>> I'd like to hear Michals opinion on that.
> 
> Well, what I can see is that this new interface is an antipatern to our
> allocation routines. We tend to control allocations by gfp mask yet you
> are introducing a bool parameter to make something faster... What that
> really means is rather arbitrary. Would it make more sense to teach
> cma_alloc resp. alloc_contig_range to recognize GFP_NOWAIT, GFP_NORETRY resp.
> GFP_RETRY_MAYFAIL instead?

Minchan did that before, but I disliked gluing things like "don't drain
lru, don't drain pcp" to GFP_NORETRY and shifting responsibility to the
user.


-- 
Thanks,

David / dhildenb