Re: [RFC 02/13] mm, page_alloc: set alloc_flags only once in slowpath

Vlastimil Babka
2016-05-31T07:59:41Z

On 05/31/2016 08:20 AM, Joonsoo Kim wrote:
>> >From 68f09f1d4381c7451238b4575557580380d8bf30 Mon Sep 17 00:00:00 2001
>> From: Vlastimil Babka <vbabka@suse.cz>
>> Date: Fri, 29 Apr 2016 11:51:17 +0200
>> Subject: [RFC 02/13] mm, page_alloc: set alloc_flags only once in slowpath
>>
>> In __alloc_pages_slowpath(), alloc_flags doesn't change after it's initialized,
>> so move the initialization above the retry: label. Also make the comment above
>> the initialization more descriptive.
>>
>> The only exception in the alloc_flags being constant is ALLOC_NO_WATERMARKS,
>> which may change due to TIF_MEMDIE being set on the allocating thread. We can
>> fix this, and make the code simpler and a bit more effective at the same time,
>> by moving the part that determines ALLOC_NO_WATERMARKS from
>> gfp_to_alloc_flags() to gfp_pfmemalloc_allowed(). This means we don't have to
>> mask out ALLOC_NO_WATERMARKS in several places in __alloc_pages_slowpath()
>> anymore. The only test for the flag can instead call gfp_pfmemalloc_allowed().
>
> Your patch looks correct to me but it makes me wonder something.
> Why do we need to mask out ALLOC_NO_WATERMARKS in several places? If
> some requestors have ALLOC_NO_WATERMARKS flag, he will
> eventually do ALLOC_NO_WATERMARKS allocation in retry loop. I don't
> understand what's the merit of masking out it.

I can think of a reason. If e.g. reclaim makes free pages above
watermark in the 4th zone in the zonelist, we would like the subsequent
get_page_from_freelist() to succeed in that 4th zone. Passing
ALLOC_NO_WATERMARKS there would likely succeed in the first zone,
needlessly below the watermark.

But this actually makes no difference, since the ALLOC_NO_WATERMARKS
attempt precedes reclaim/compaction attempts. It probably shouldn't...

> Thanks.
>