Messages in this thread | | | Date | Fri, 6 Jan 2017 11:18:44 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: [PATCH v2] mm: Respect FOLL_FORCE/FOLL_COW for thp |
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On Thu, Jan 05, 2017 at 08:50:25PM -0500, Keno Fischer wrote: > In 19be0eaff ("mm: remove gup_flags FOLL_WRITE games from __get_user_pages()"), > the mm code was changed from unsetting FOLL_WRITE after a COW was resolved to > setting the (newly introduced) FOLL_COW instead. Simultaneously, the check in > gup.c was updated to still allow writes with FOLL_FORCE set if FOLL_COW had > also been set. However, a similar check in huge_memory.c was forgotten. As a > result, remote memory writes to ro regions of memory backed by transparent huge > pages cause an infinite loop in the kernel (handle_mm_fault sets FOLL_COW and > returns 0 causing a retry, but follow_trans_huge_pmd bails out immidiately > because `(flags & FOLL_WRITE) && !pmd_write(*pmd)` is true. While in this > state the process is stil SIGKILLable, but little else works (e.g. no ptrace > attach, no other signals). This is easily reproduced with the following > code (assuming thp are set to always): > > #include <assert.h> > #include <fcntl.h> > #include <stdint.h> > #include <stdio.h> > #include <string.h> > #include <sys/mman.h> > #include <sys/stat.h> > #include <sys/types.h> > #include <sys/wait.h> > #include <unistd.h> > > #define TEST_SIZE 5 * 1024 * 1024 > > int main(void) { > int status; > pid_t child; > int fd = open("/proc/self/mem", O_RDWR); > void *addr = mmap(NULL, TEST_SIZE, PROT_READ, > MAP_ANONYMOUS | MAP_PRIVATE, 0, 0); > assert(addr != MAP_FAILED); > pid_t parent_pid = getpid(); > if ((child = fork()) == 0) { > void *addr2 = mmap(NULL, TEST_SIZE, PROT_READ | PROT_WRITE, > MAP_ANONYMOUS | MAP_PRIVATE, 0, 0); > assert(addr2 != MAP_FAILED); > memset(addr2, 'a', TEST_SIZE); > pwrite(fd, addr2, TEST_SIZE, (uintptr_t)addr); > return 0; > } > assert(child == waitpid(child, &status, 0)); > assert(WIFEXITED(status) && WEXITSTATUS(status) == 0); > return 0; > } > > Fix this by updating follow_trans_huge_pmd in huge_memory.c analogously to > the update in gup.c in the original commit. The same pattern exists in > follow_devmap_pmd. However, we should not be able to reach that check > with FOLL_COW set, so add WARN_ONCE to make sure we notice if we ever > do. > > Signed-off-by: Keno Fischer <keno@juliacomputing.com>
Cc: stable@ ?
Acke-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
-- Kirill A. Shutemov
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