linux-kernel - Re: [PATCH RFC] dma-direct: do not allocate a single page from CMA area

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Message-ID: <20181105224050.GA10411@Asurada-Nvidia.nvidia.com>
Date:   Mon, 5 Nov 2018 14:40:51 -0800
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Robin Murphy <robin.murphy@....com>, m.szyprowski@...sung.com,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        vdumpa@...dia.com
Subject: Re: [PATCH RFC] dma-direct: do not allocate a single page from CMA
 area

On Fri, Nov 02, 2018 at 07:35:42AM +0100, Christoph Hellwig wrote:
> On Thu, Nov 01, 2018 at 02:07:55PM +0000, Robin Murphy wrote:
> > On 31/10/2018 20:03, Nicolin Chen wrote:
> >> The addresses within a single page are always contiguous, so it's
> >> not so necessary to allocate one single page from CMA area. Since
> >> the CMA area has a limited predefined size of space, it might run
> >> out of space in some heavy use case, where there might be quite a
> >> lot CMA pages being allocated for single pages.
> >>
> >> This patch tries to skip CMA allocations of single pages and lets
> >> them go through normal page allocations. This would save resource
> >> in the CMA area for further more CMA allocations.
> >
> > In general, this seems to make sense to me. It does represent a theoretical 
> > change in behaviour for devices which have their own CMA area somewhere 
> > other than kernel memory, and only ever make non-atomic allocations, but 
> > I'm not sure whether that's a realistic or common enough case to really 
> > worry about.
> 
> Yes, I think we should make the decision in dma_alloc_from_contiguous
> based on having a per-dev CMA area or not.  There is a lot of cruft in

It seems that cma_alloc() already has a CMA area check? Would it
be duplicated to have a similar one in dma_alloc_from_contiguous?

> this area that should be cleaned up while we're at it, like always
> falling back to the normal page allocator if there is no CMA area or
> nothing suitable found in dma_alloc_from_contiguous instead of
> having to duplicate all that in the caller.

Am I supposed to clean up things that's mentioned above by moving
the fallback allocator into dma_alloc_from_contiguous, or to just
move my change (the count check) into dma_alloc_from_contiguous?

I understand that'd be great to have a cleanup, yet feel it could
be done separately as this patch isn't really a cleanup change.

Thanks
Nicolin