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

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Date:   Mon, 19 Nov 2018 18:39:08 -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

Robin? Christ?

On Mon, Nov 05, 2018 at 02:40:50PM -0800, Nicolin Chen wrote:
> 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