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

[Vlastimil Babka]

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...

> Thanks.