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

Joonsoo Kim
2016-05-31T06:19:53Z

On Tue, May 10, 2016 at 02:30:11PM +0200, Vlastimil Babka wrote:
> On 05/10/2016 01:28 PM, Tetsuo Handa wrote:
> > Vlastimil Babka wrote:
> >> 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.
> >
> > Not true. gfp_to_alloc_flags() will include ALLOC_NO_WATERMARKS if current
> > thread got TIF_MEMDIE after gfp_to_alloc_flags() was called for the first
>
> Oh, right. Stupid global state.
>
> > time. Do you want to make TIF_MEMDIE threads fail their allocations without
> > using memory reserves?
>
> No, thanks for catching this. How about the following version? I think
> that's even nicer cleanup, if correct. Note it causes a conflict in
> patch 03/13 but it's simple to resolve.
>
> Thanks
>
> ----8<----
> >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.

Thanks.