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Date:   Fri, 20 Apr 2018 08:20:23 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@redhat.com>
To:     Matthew Wilcox <willy@infradead.org>
cc:     David Miller <davem@davemloft.net>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        eric.dumazet@gmail.com, edumazet@google.com,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        mst@redhat.com, jasowang@redhat.com,
        virtualization@lists.linux-foundation.org, dm-devel@redhat.com,
        Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH] kvmalloc: always use vmalloc if CONFIG_DEBUG_VM
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On Fri, 20 Apr 2018, Matthew Wilcox wrote:

> On Thu, Apr 19, 2018 at 12:12:38PM -0400, Mikulas Patocka wrote:
> > Unfortunatelly, some kernel code has bugs - it uses kvmalloc and then
> > uses DMA-API on the returned memory or frees it with kfree. Such bugs were
> > found in the virtio-net driver, dm-integrity or RHEL7 powerpc-specific
> > code.
> 
> Maybe it's time to have the SG code handle vmalloced pages?  This is
> becoming more and more common with vmapped stacks (and some of our
> workarounds are hideous -- allocate 4 bytes with kmalloc because we can't
> DMA onto the stack any more?).  We already have a few places which do
> handle sgs of vmalloced addresses, such as the nx crypto driver:
> 
>         if (is_vmalloc_addr(start_addr))
>                 sg_addr = page_to_phys(vmalloc_to_page(start_addr))
>                           + offset_in_page(sg_addr);
>         else
>                 sg_addr = __pa(sg_addr);
> 
> and videobuf:
> 
>                 pg = vmalloc_to_page(virt);
>                 if (NULL == pg)
>                         goto err;
>                 BUG_ON(page_to_pfn(pg) >= (1 << (32 - PAGE_SHIFT)));
>                 sg_set_page(&sglist[i], pg, PAGE_SIZE, 0);
> 
> Yes, there's the potential that we have to produce two SG entries for a
> virtually contiguous region if it crosses a page boundary, and our APIs
> aren't set up right to make it happen.  But this is something we should
> consider fixing ... otherwise we'll end up with dozens of driver hacks.
> The videobuf implementation was already copy-and-pasted into the saa7146
> driver, for example.

What if the device requires physically contiguous area and the vmalloc 
area crosses a page? Will you use a bounce buffer? Where do you allocate 
the bounce buffer from? What if you run out of bounce buffers?

Mikulkas