Subject: Re: [PATCH 2/9] mm/page_alloc: Add a bulk page allocator

From: Mel Gorman <[email-masked]>
Date: Mon, 12 Apr 2021 12:19:51 +0100

On Mon, Apr 12, 2021 at 11:59:38AM +0100, Mel Gorman wrote:
> > I don't understand this comment. Only alloc_flags_nofragment() sets this flag
> > and we don't use it here?
> > 
> 
> It's there as a reminder that there are non-obvious consequences
> to ALLOC_NOFRAGMENT that may affect the bulk allocation success
> rate. __rmqueue_fallback will only select pageblock_order pages and if that
> fails, we fall into the slow path that allocates a single page. I didn't
> deal with it because it was not obvious that it's even relevant but I bet
> in 6 months time, I'll forget that ALLOC_NOFRAGMENT may affect success
> rates without the comment. I'm waiting for a bug that can trivially trigger
> a case with a meaningful workload where the success rate is poor enough to
> affect latency before adding complexity. Ideally by then, the allocation
> paths would be unified a bit better.
> 

So this needs better clarification. ALLOC_NOFRAGMENT is not a
problem at the moment but at one point during development, it was a
non-obvious potential problem. If the paths are unified, ALLOC_NOFRAGMENT
*potentially* becomes a problem depending on how it's done and it needs
careful consideration. For example, it could be part unified by moving
the alloc_flags_nofragment() call into prepare_alloc_pages because in
__alloc_pages, it always happens and it looks like an obvious partial
unification. Hence the comment "May set ALLOC_NOFRAGMENT" because I wanted
a reminder in case I "fixed" this in 6 months time and forgot the downside.

-- 
Mel Gorman
SUSE Labs