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

[T.J. Mercier]

On Thu, Jan 18, 2024 at 6:49 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
>
> 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.

I actually had it in my head that it was a BUG_ON check for VM_PFNMAP
in dma_buf_mmap and it was merged, so I was surprised to discover that
it wasn't set for these CMA buffers.

> 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.

Looking back at the patch, the CI email at the end of the thread lists
a bunch of now-broken links to DMESG-WARN test failures I assume
pointed at a large chunk of them.

https://lore.kernel.org/all/166919750173.15575.2864736980735346730@xxxxxxxxxxxxxxxxxxxxxx/

> >
> > > [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>

Thanks Christian.