Re: [PATCH] dma-buf: heaps: Don't track CMA dma-buf pages under RssFile

[Daniel Vetter]

On Thu, Jan 18, 2024 at 11:02:22AM +0100, Christian König wrote:
> Am 17.01.24 um 19:11 schrieb T.J. Mercier:
> > DMA buffers allocated from the CMA dma-buf heap get counted under
> > RssFile for processes that map them and trigger page faults. In
> > addition to the incorrect accounting reported to userspace, reclaim
> > behavior was influenced by the MM_FILEPAGES counter until linux 6.8, but
> > this memory is not reclaimable. [1] Change the CMA dma-buf heap to set
> > VM_PFNMAP on the VMA so MM does not poke at the memory managed by this
> > dma-buf heap, and use vmf_insert_pfn to correct the RSS accounting.
> > 
> > The system dma-buf heap does not suffer from this issue since
> > remap_pfn_range is used during the mmap of the buffer, which also sets
> > VM_PFNMAP on the VMA.
> 
> Mhm, not an issue with this patch but Daniel wanted to add a check for
> VM_PFNMAP to dma_buf_mmap() which would have noted this earlier.
> 
> I don't fully remember the discussion but for some reason that was never
> committed. We should probably try that again.

Iirc the issue is that dma_mmap is not guaranteed to give you a VM_SPECIAL
mapping, at least on absolutely all architectures. That's why I defacto
dropped that idea, but it would indeed be really great if we could
resurrect it.

Maybe for x86 only? Or x86+armv8, I'm honestly not sure anymore which
exact cases ended up with a VM_NORMAL mapping ... Would need a pile of
digging.

I think all the other patches I've landed.
-Sima

> 
> > [1]https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/mm/vmscan.c?id=fb46e22a9e3863e08aef8815df9f17d0f4b9aede
> > 
> > Fixes: b61614ec318a ("dma-buf: heaps: Add CMA heap to dmabuf heaps")
> > Signed-off-by: T.J. Mercier<tjmercier@xxxxxxxxxx>
> 
> Acked-by: Christian König <christian.koenig@xxxxxxx>
> 
> > ---
> >   drivers/dma-buf/heaps/cma_heap.c | 7 +++----
> >   1 file changed, 3 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/dma-buf/heaps/cma_heap.c b/drivers/dma-buf/heaps/cma_heap.c
> > index ee899f8e6721..4a63567e93ba 100644
> > --- a/drivers/dma-buf/heaps/cma_heap.c
> > +++ b/drivers/dma-buf/heaps/cma_heap.c
> > @@ -168,10 +168,7 @@ static vm_fault_t cma_heap_vm_fault(struct vm_fault *vmf)
> >   	if (vmf->pgoff > buffer->pagecount)
> >   		return VM_FAULT_SIGBUS;
> > -	vmf->page = buffer->pages[vmf->pgoff];
> > -	get_page(vmf->page);
> > -
> > -	return 0;
> > +	return vmf_insert_pfn(vma, vmf->address, page_to_pfn(buffer->pages[vmf->pgoff]));
> >   }
> >   static const struct vm_operations_struct dma_heap_vm_ops = {
> > @@ -185,6 +182,8 @@ static int cma_heap_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma)
> >   	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0)
> >   		return -EINVAL;
> > +	vm_flags_set(vma, VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP);
> > +
> >   	vma->vm_ops = &dma_heap_vm_ops;
> >   	vma->vm_private_data = buffer;

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch