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

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Message-ID: <20181120092010.GA7270@lst.de>
Date:   Tue, 20 Nov 2018 10:20:10 +0100
From:   Christoph Hellwig <hch@....de>
To:     Nicolin Chen <nicoleotsuka@...il.com>
Cc:     Christoph Hellwig <hch@....de>,
        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 Mon, Nov 05, 2018 at 02:40:51PM -0800, Nicolin Chen wrote:
> > > 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?

It isn't duplicate if it serves a different purpose.

> > 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.

I can take care of any cleanups.  I've been trying to dust up that
area anyway.