From mboxrd@z Thu Jan  1 00:00:00 1970
From: Matthew Wilcox <willy-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
Subject: Re: [PATCH v2 6/9] dmapool: improve scalability of dma_pool_free
Date: Fri, 3 Aug 2018 14:02:15 -0700
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To: Tony Battersby <tonyb-vFAe+i1/wJI5UWNf+nJyDw@public.gmane.org>
Cc: linux-scsi-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Chaitra P B <chaitra.basappa-dY08KVG/lbpWk0Htik3J/w@public.gmane.org>, Suganath Prabu Subramani <suganath-prabu.subramani-dY08KVG/lbpWk0Htik3J/w@public.gmane.org>, Sathya Prakash <sathya.prakash-dY08KVG/lbpWk0Htik3J/w@public.gmane.org>, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, MPT-FusionLinux.pdl-dY08KVG/lbpWk0Htik3J/w@public.gmane.org, Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>
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On Fri, Aug 03, 2018 at 04:05:35PM -0400, Tony Battersby wrote:
> For v3 of the patchset, I was also considering to add a note to the
> kernel-doc comments for dma_pool_create() to use dma_alloc_coherent()
> directly instead of a dma pool if the driver intends to allow userspace
> to mmap() the returned pages, due to the new use of the _mapcount union
> in struct page.=A0 Would you consider that useful information or pointless
> trivia?

If userspace is going to map the pages, it's going to expose other things
to userspace than the dma pages.  I'd suggest they not do this; they
should do their own sub-allocation which only exposes to an individual
task the data they're sure is OK for each task to see.