linux-kernel - Re: [RFC PATCH 2/2] mm,drm/ttm: Use VM

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Date:   Tue, 23 Mar 2021 11:04:45 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Thomas Hellström (Intel) 
        <thomas_os@...pmail.org>, dri-devel@...ts.freedesktop.org,
        Christian Koenig <christian.koenig@....com>,
        David Airlie <airlied@...ux.ie>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] mm,drm/ttm: Use VM_PFNMAP for TTM vmas

On Tue, Mar 23, 2021 at 12:47:24PM +0100, Daniel Vetter wrote:

> > +static inline bool is_cow_mapping(vm_flags_t flags)
> 
> Bit a bikeshed, but I wonder whether the public interface shouldn't be
> vma_is_cow_mapping. Or whether this shouldn't be rejected somewhere else,
> since at least in drivers/gpu we have tons of cases that don't check for
> this and get it all kinds of wrong I think.
> 
> remap_pfn_range handles this for many cases, but by far not for all.
> 
> Anyway patch itself lgtm:
> 
> Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>

I would like it if io_remap_pfn_range() did not allow shared mappings
at all.

IIRC it doesn't work anyway, the kernel can't reliably copy from IO
pages eg the "_copy_from_user_inatomic()" under cow_user_page() will
not work on s390 that requires all IO memory be accessed with special
instructions.

Unfortunately I have no idea what the long ago special case of
allowing COW'd IO mappings is. :\

Jason