Re: [PATCH 1/1] dma-buf: heaps: Map system heap pages as managed by linux vm

From: Christoph Hellwig <hch@infradead.org>
To: Suren Baghdasaryan <surenb@google.com>
Cc: sumit.semwal@linaro.org, benjamin.gaignard@linaro.org,
	lmark@codeaurora.org, labbott@redhat.com, Brian.Starkey@arm.com,
	john.stultz@linaro.org, christian.koenig@amd.com,
	cgoldswo@codeaurora.org, orjan.eide@arm.com,
	robin.murphy@arm.com, jajones@nvidia.com, minchan@kernel.org,
	hridya@google.com, sspatil@google.com,
	linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org,
	linaro-mm-sig@lists.linaro.org, linux-kernel@vger.kernel.org,
	kernel-team@android.com
Subject: Re: [PATCH 1/1] dma-buf: heaps: Map system heap pages as managed by linux vm
Date: Thu, 28 Jan 2021 09:13:48 +0000	[thread overview]
Message-ID: <20210128091348.GA1962975@infradead.org> (raw)
In-Reply-To: <20210128083817.314315-1-surenb@google.com>

On Thu, Jan 28, 2021 at 12:38:17AM -0800, Suren Baghdasaryan wrote:
> Currently system heap maps its buffers with VM_PFNMAP flag using
> remap_pfn_range. This results in such buffers not being accounted
> for in PSS calculations because vm treats this memory as having no
> page structs. Without page structs there are no counters representing
> how many processes are mapping a page and therefore PSS calculation
> is impossible.
> Historically, ION driver used to map its buffers as VM_PFNMAP areas
> due to memory carveouts that did not have page structs [1]. That
> is not the case anymore and it seems there was desire to move away
> from remap_pfn_range [2].
> Dmabuf system heap design inherits this ION behavior and maps its
> pages using remap_pfn_range even though allocated pages are backed
> by page structs.
> Clear VM_IO and VM_PFNMAP flags when mapping memory allocated by the
> system heap and replace remap_pfn_range with vm_insert_page, following
> Laura's suggestion in [1]. This would allow correct PSS calculation
> for dmabufs.
> 
> [1] https://driverdev-devel.linuxdriverproject.narkive.com/v0fJGpaD/using-ion-memory-for-direct-io
> [2] http://driverdev.linuxdriverproject.org/pipermail/driverdev-devel/2018-October/127519.html
> (sorry, could not find lore links for these discussions)
> 
> Suggested-by: Laura Abbott <labbott@kernel.org>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
>  drivers/dma-buf/heaps/system_heap.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/dma-buf/heaps/system_heap.c b/drivers/dma-buf/heaps/system_heap.c
> index 17e0e9a68baf..0e92e42b2251 100644
> --- a/drivers/dma-buf/heaps/system_heap.c
> +++ b/drivers/dma-buf/heaps/system_heap.c
> @@ -200,11 +200,13 @@ static int system_heap_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma)
>  	struct sg_page_iter piter;
>  	int ret;
>  
> +	/* All pages are backed by a "struct page" */
> +	vma->vm_flags &= ~VM_PFNMAP;

Why do we clear this flag?  It shouldn't even be set here as far as I
can tell.