From: Nicolin Chen <nicoleotsuka@gmail.com>
To: Robin Murphy <robin.murphy@arm.com>
Cc: hch@lst.de, m.szyprowski@samsung.com,
iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
vdumpa@nvidia.com
Subject: Re: [PATCH RFC] dma-direct: do not allocate a single page from CMA area
Date: Thu, 1 Nov 2018 11:04:40 -0700 [thread overview]
Message-ID: <20181101180439.GA4746@Asurada-Nvidia.nvidia.com> (raw)
In-Reply-To: <13d60076-33ad-b542-4d17-4d717d5aa4d3@arm.com>
Hi Robin,
Thanks for the comments.
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.
Hmm..I don't quite understand the part of worrying its realisticness.
Would you mind elaborating a bit? As I tested this change on Tegra186
board, and saw some single-page allocations have been directed to the
normal allocation; and the "CmaFree" size reported from /proc/meminfo
is also increased. Does this mean it's realistic?
Thank you
Nicolin
-----
>
> Robin.
>
> > Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
> > ---
> > kernel/dma/direct.c | 8 ++++++--
> > 1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> > index 22a12ab5a5e9..14c5d49eded2 100644
> > --- a/kernel/dma/direct.c
> > +++ b/kernel/dma/direct.c
> > @@ -120,8 +120,12 @@ void *dma_direct_alloc_pages(struct device *dev, size_t size,
> > gfp |= __dma_direct_optimal_gfp_mask(dev, dev->coherent_dma_mask,
> > &phys_mask);
> > again:
> > - /* CMA can be used only in the context which permits sleeping */
> > - if (gfpflags_allow_blocking(gfp)) {
> > + /*
> > + * CMA can be used only in the context which permits sleeping.
> > + * Since addresses within one PAGE are always contiguous, skip
> > + * CMA allocation for a single page to save CMA reserved space
> > + */
> > + if (gfpflags_allow_blocking(gfp) && count > 1) {
> > page = dma_alloc_from_contiguous(dev, count, page_order,
> > gfp & __GFP_NOWARN);
> > if (page && !dma_coherent_ok(dev, page_to_phys(page), size)) {
> >
next prev parent reply other threads:[~2018-11-01 18:04 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-31 20:03 [PATCH RFC] dma-direct: do not allocate a single page from CMA area Nicolin Chen
2018-11-01 14:07 ` Robin Murphy
2018-11-01 18:04 ` Nicolin Chen [this message]
2018-11-01 19:32 ` Robin Murphy
2018-11-01 20:22 ` Nicolin Chen
2018-11-02 6:35 ` Christoph Hellwig
2018-11-05 22:40 ` Nicolin Chen
2018-11-20 2:39 ` Nicolin Chen
2018-11-20 9:20 ` Christoph Hellwig
2018-11-21 1:30 ` Nicolin Chen
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