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Date:   Mon, 12 Apr 2021 12:19:51 +0100
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Chuck Lever <chuck.lever@oracle.com>,
        Jesper Dangaard Brouer <brouer@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        Alexander Duyck <alexander.duyck@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        Ilias Apalodimas <ilias.apalodimas@linaro.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-Net <netdev@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Linux-NFS <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH 2/9] mm/page_alloc: Add a bulk page allocator
Message-ID: <20210412111951.GW3697@techsingularity.net>
References: <20210325114228.27719-1-mgorman@techsingularity.net>
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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