linux-kernel - Re: [PATCH v2 RFC/RFT 0/5] Save single pages from CMA area

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Date:   Wed, 27 Mar 2019 11:42:32 -0700
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     robin.murphy@....com, vdumpa@...dia.com, linux@...linux.org.uk,
        catalin.marinas@....com, will.deacon@....com, joro@...tes.org,
        m.szyprowski@...sung.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        tony@...mide.com
Subject: Re: [PATCH v2 RFC/RFT 0/5] Save single pages from CMA area

On Wed, Mar 27, 2019 at 09:08:21AM +0100, Christoph Hellwig wrote:
> On Tue, Mar 26, 2019 at 04:01:26PM -0700, Nicolin Chen wrote:
> > This series of patches try to save single pages from CMA area bypassing
> > all CMA single page alloctions and allocating normal pages instead, as
> > all addresses within one single page are contiguous.
> > 
> > We had once applied the PATCH-5 but reverted it as actually not all the
> > callers handled the fallback allocations. Per Robin's suggestion, let's
> > stuff alloc_pages()/free_page() fallbacks to those callers before having
> > PATCH-5.
> 
> Given the problems this has caused so far I'd like to see a good
> explanation of why this optimization is so important that all the churn
> is even worth it..

With certain downstream user cases, we had run into a CMA exhaustion
situation, and this was one of the changes that eased the problem. I
have all the reasoning in the PATCH-5 commit message. And what Robin
tested can also justify for it:
https://lore.kernel.org/patchwork/patch/1004934/#1190139

Thanks