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

From: Vlastimil Babka
Date: Tue May 31 2016 - 03:59:45 EST

On 05/31/2016 08:20 AM, Joonsoo Kim wrote:
>From 68f09f1d4381c7451238b4575557580380d8bf30 Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@xxxxxxx>
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...