On Thu, Jan 18, 2024 at 6:49 AM Daniel Vetter daniel@ffwll.ch 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@emeril.fr...
[1]https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/mm...
Fixes: b61614ec318a ("dma-buf: heaps: Add CMA heap to dmabuf heaps") Signed-off-by: T.J. Merciertjmercier@google.com
Acked-by: Christian König christian.koenig@amd.com
Thanks Christian.