Subject: Re: [RFC V3] net: don't wait for order-3 page allocation

From: Vlastimil Babka <[email-masked]>
Date: Thu, 18 Jun 2015 17:22:40 +0200

On 06/18/2015 04:43 PM, Michal Hocko wrote:
> On Thu 18-06-15 07:35:53, Eric Dumazet wrote:
>> On Thu, Jun 18, 2015 at 7:30 AM, Michal Hocko <[email-masked]> wrote:
>>
>>> Abusing __GFP_NO_KSWAPD is a wrong way to go IMHO. It is true that the
>>> _current_ implementation of the allocator has this nasty and very subtle
>>> side effect but that doesn't mean it should be abused outside of the mm
>>> proper. Why shouldn't this path wake the kswapd and let it compact
>>> memory on the background to increase the success rate for the later
>>> high order allocations?
>>
>> I kind of agree.
>>
>> If kswapd is a problem (is it ???) we should fix it, instead of adding
>> yet another flag to some random locations attempting
>> memory allocations.
>
> No, kswapd is not a problem. The problem is ~__GFP_WAIT allocation can
> access some portion of the memory reserves (see gfp_to_alloc_flags resp.
> __zone_watermark_ok and ALLOC_HARDER). __GFP_NO_KSWAPD is just a dirty
> hack to not give that access which was introduced for THP AFAIR.
>
> The implicit access to memory reserves for non sleeping allocation has
> been there for ages and it might be not suitable for this particular
> path but that doesn't mean another gfp flag with a different side effect
> should be hijacked. We should either stop doing that implicit access to
> memory reserves and give __GFP_RESERVE or add the __GFP_NORESERVE. But
> that is a problem to be solved in the mm proper. Spreading subtle
> dependencies outside of mm will just make situation worse.

So you are not proposing to use these __GFP_RESERVE/NORESERVE flag 
outside of mm, right? (besides, we distinguish several kinds of 
reserves, so what exactly would the flag do?) As that would be also 
subtle dependency. The general problem I think is that we should want 
the mm users to specify higher-level intentions (such as GFP_KERNEL) 
which would map to specific directions (__GFP_*) for the allocator, and 
currently it's rather a mess of both kinds of flags. Clearly the 
intention here is "opportunistic allocation that should not 
reclaim/compact, use reserves, wake up kswapd (?) because it's better to 
fall back to smaller pages than wait") and we don't seem to have a 
GFP_OPPORTUNISTIC flag for that. The allocation has to then mask out 
__GFP_WAIT which however looks like an atomic allocation to the 
allocator and give access to reserves, etc...

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