On Wed, Feb 13, 2019 at 9:59 PM Andrew Morton <a...@linux-foundation.org> wrote: > > On Wed, 13 Feb 2019 21:41:57 +0100 Jann Horn <ja...@google.com> wrote: > > > The basic idea behind ->pagecnt_bias is: If we pre-allocate the maximum > > number of references that we might need to create in the fastpath later, > > the bump-allocation fastpath only has to modify the non-atomic bias value > > that tracks the number of extra references we hold instead of the atomic > > refcount. The maximum number of allocations we can serve (under the > > assumption that no allocation is made with size 0) is nc->size, so that's > > the bias used. > > > > However, even when all memory in the allocation has been given away, a > > reference to the page is still held; and in the `offset < 0` slowpath, the > > page may be reused if everyone else has dropped their references. > > This means that the necessary number of references is actually > > `nc->size+1`. > > > > Luckily, from a quick grep, it looks like the only path that can call > > page_frag_alloc(fragsz=1) is TAP with the IFF_NAPI_FRAGS flag, which > > requires CAP_NET_ADMIN in the init namespace and is only intended to be > > used for kernel testing and fuzzing. > > For the net-naive, what is TAP? It doesn't appear to mean > drivers/net/tap.c.
It's implemented in drivers/net/tun.c; the combined functionality implemented in there is called TUN/TAP. TUN refers to providing raw IP packets to the kernel, TAP refers to providing raw ethernet packets. It's documented in Documentation/networking/tuntap.txt. The code that's interesting here is tun_get_user(), which calls into tun_napi_alloc_frags() if tun_napi_frags_enabled(tfile) is true, which in turn calls into netdev_alloc_frag(), which ends up in page_frag_alloc(). This is how you can use it (except that if you were using it legitimately, you'd be writing an ethernet header, a layer 3 header, and application data instead of writing "aaaaaaaaaaaaaaa" like me): ================ #define _GNU_SOURCE #include <stdlib.h> #include <stdarg.h> #include <net/if.h> #include <linux/if.h> #include <linux/if_tun.h> #include <err.h> #include <sys/types.h> #include <fcntl.h> #include <string.h> #include <stdio.h> #include <unistd.h> #include <sys/ioctl.h> void systemf(const char *command, ...) { char *full_command; va_list ap; va_start(ap, command); if (vasprintf(&full_command, command, ap) == -1) err(1, "vasprintf"); va_end(ap); printf("systemf: <<<%s>>>\n", full_command); system(full_command); } char *devname; int tun_alloc(char *name) { int fd = open("/dev/net/tun", O_RDWR); if (fd == -1) err(1, "open tun dev"); static struct ifreq req = { .ifr_flags = IFF_TAP|IFF_NO_PI|IFF_NAPI_FRAGS|IFF_NAPI }; strcpy(req.ifr_name, name); if (ioctl(fd, TUNSETIFF, &req)) err(1, "TUNSETIFF"); devname = req.ifr_name; printf("device name: %s\n", devname); return fd; } int main(void) { int tun_fd = tun_alloc("inject_dev%d"); systemf("ip link set %s up", devname); while (1) { struct iovec iov[15]; for (int i=0; i<sizeof(iov)/sizeof(iov[0]); i++) { iov[i].iov_base = "a"; iov[i].iov_len = 1; } writev(tun_fd, iov, sizeof(iov)/sizeof(iov[0])); } } ================ > > To test for this issue, put a `WARN_ON(page_ref_count(page) == 0)` in the > > `offset < 0` path, below the virt_to_page() call, and then repeatedly call > > writev() on a TAP device with IFF_TAP|IFF_NO_PI|IFF_NAPI_FRAGS|IFF_NAPI, > > with a vector consisting of 15 elements containing 1 byte each. > > > > ... > > > > --- a/mm/page_alloc.c > > +++ b/mm/page_alloc.c > > @@ -4675,11 +4675,11 @@ void *page_frag_alloc(struct page_frag_cache *nc, > > /* Even if we own the page, we do not use atomic_set(). > > * This would break get_page_unless_zero() users. > > */ > > - page_ref_add(page, size - 1); > > + page_ref_add(page, size); > > > > /* reset page count bias and offset to start of new frag */ > > nc->pfmemalloc = page_is_pfmemalloc(page); > > - nc->pagecnt_bias = size; > > + nc->pagecnt_bias = size + 1; > > nc->offset = size; > > } > > > > @@ -4695,10 +4695,10 @@ void *page_frag_alloc(struct page_frag_cache *nc, > > size = nc->size; > > #endif > > /* OK, page count is 0, we can safely set it */ > > - set_page_count(page, size); > > + set_page_count(page, size + 1); > > > > /* reset page count bias and offset to start of new frag */ > > - nc->pagecnt_bias = size; > > + nc->pagecnt_bias = size + 1; > > offset = size - fragsz; > > } > > This is probably more a davem patch than a -mm one. Ah, sorry. I assumed that I just should go by which directory the patched code is in. You did just add it to the -mm tree though, right? So I shouldn't resend it to davem?