On Mon, 23 Apr 2018, David Rientjes wrote: > On Mon, 23 Apr 2018, Mikulas Patocka wrote: > > > The kvmalloc function tries to use kmalloc and falls back to vmalloc if > > kmalloc fails. > > > > 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. > > > > These bugs are hard to reproduce because kvmalloc falls back to vmalloc > > only if memory is fragmented. > > > > In order to detect these bugs reliably I submit this patch that changes > > kvmalloc to fall back to vmalloc with 1/2 probability if CONFIG_DEBUG_SG > > is turned on. CONFIG_DEBUG_SG is used, because it makes the DMA API layer > > verify the addresses passed to it, and so the user will get a reliable > > stacktrace. > > Why not just do it unconditionally? Sounds better than "50% of the time > this will catch bugs". Because kmalloc (with slub_debug) detects buffer overflows better than vmalloc. vmalloc detects buffer overflows only at a page boundary. This is intended for debugging kernels and debugging kernels should detect as many bugs as possible. Mikulas
