Re: [PATCH 10 of 20] ipath - support for userspace apps using coredriver

[Nick Piggin]

Hugh Dickins wrote:
> On Thu, 16 Mar 2006, Nick Piggin wrote:
>
> > > How about the case where one wants to map pages from
> > > dma_alloc_coherent() into userspace?  It seems one should do
> > > get_page() in .nopage, and then the driver can do dma_free_coherent()
> > > when the vma is released.
> >
> > I think so, provided you set VM_IO on the vma. You need VM_IO to
> > ensure that get_user_pages callers can't hijack your page's lifetime
> > rules
>
>
> Once __GFP_COMP is passed to the dma_alloc_coherent, as it needs to be
> (unless going VM_PFNMAP), get_user_pages will be safe: no need for VM_IO.

But it doesn't look like dma_alloc_coherent is guaranteed to return
memory allocated from the regular page allocator, nor even memory
backed by a struct page.

For example, I see one that returns kmalloc()ed memory. If the pages
for the slab are already allocated then __GFP_COMP will not do anything
there. i386 looks like it has a path that uses ioremap...

Now I haven't looked through all these closely like you will have, but
I'd like to know how __GFP_COMP solves all the potential problems I
see.

--
SUSE Labs, Novell Inc.
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