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
