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    SubjectRe: [PATCH] kvmalloc: always use vmalloc if CONFIG_DEBUG_VM
    On Fri, Apr 20, 2018 at 08:20:23AM -0400, Mikulas Patocka wrote:
    >
    >
    > 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

    I agree with Matthew here.

    4 byte variables are typically size aligned so won't cross a boundary.

    That's enough for virtio at least. People using structs can force
    alignment.

    We could wrap access in a macro (sizeof(x) >= alignof(x)) to help
    guarantee that.

    --
    MST

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