A few new fields were added to mmu_gather to make TLB flush smarter for huge page by telling what level of page table is changed.

__tlb_reset_range() is used to reset all these page table state to unchanged, which is called by TLB flush for parallel mapping changes for the same range under non-exclusive lock (i.e. read mmap_sem). Before commit dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap"), the syscalls (e.g. MADV_DONTNEED, MADV_FREE) which may update PTEs in parallel don't remove page tables. But, the forementioned commit may do munmap() under read mmap_sem and free page tables. This may result in program hang on aarch64 reported by Jan Stancek. The problem could be reproduced by his test program with slightly modified below.

---8<---

static int map_size = 4096; static int num_iter = 500; static long threads_total;

static void *distant_area;

void *map_write_unmap(void *ptr) { int *fd = ptr; unsigned char *map_address; int i, j = 0;

for (i = 0; i < num_iter; i++) { map_address = mmap(distant_area, (size_t) map_size, PROT_WRITE | PROT_READ, MAP_SHARED | MAP_ANONYMOUS, -1, 0); if (map_address == MAP_FAILED) { perror("mmap"); exit(1); }

for (j = 0; j < map_size; j++) map_address[j] = 'b';

if (munmap(map_address, map_size) == -1) { perror("munmap"); exit(1); } }

return NULL; }

void *dummy(void *ptr) { return NULL; }

int main(void) { pthread_t thid[2];

/* hint for mmap in map_write_unmap() */ distant_area = mmap(0, DISTANT_MMAP_SIZE, PROT_WRITE | PROT_READ, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0); munmap(distant_area, (size_t)DISTANT_MMAP_SIZE); distant_area += DISTANT_MMAP_SIZE / 2;

while (1) { pthread_create(&thid[0], NULL, map_write_unmap, NULL); pthread_create(&thid[1], NULL, dummy, NULL);

pthread_join(thid[0], NULL); pthread_join(thid[1], NULL); } } ---8<---

The program may bring in parallel execution like below:

t1 t2 munmap(map_address) downgrade_write(&mm->mmap_sem); unmap_region() tlb_gather_mmu() inc_tlb_flush_pending(tlb->mm); free_pgtables() tlb->freed_tables = 1 tlb->cleared_pmds = 1

pthread_exit() madvise(thread_stack, 8M, MADV_DONTNEED) zap_page_range() tlb_gather_mmu() inc_tlb_flush_pending(tlb->mm);

tlb_finish_mmu() if (mm_tlb_flush_nested(tlb->mm)) __tlb_reset_range()

__tlb_reset_range() would reset freed_tables and cleared_* bits, but this may cause inconsistency for munmap() which do free page tables. Then it may result in some architectures, e.g. aarch64, may not flush TLB completely as expected to have stale TLB entries remained.

Use fullmm flush since it yields much better performance on aarch64 and non-fullmm doesn't yields significant difference on x86.

The original proposed fix came from Jan Stancek who mainly debugged this issue, I just wrapped up everything together.

Reported-by: Jan Stancek jstancek@redhat.com Tested-by: Jan Stancek jstancek@redhat.com Suggested-by: Will Deacon will.deacon@arm.com Cc: Peter Zijlstra peterz@infradead.org Cc: Nick Piggin npiggin@gmail.com Cc: "Aneesh Kumar K.V" aneesh.kumar@linux.ibm.com Cc: Nadav Amit namit@vmware.com Cc: Minchan Kim minchan@kernel.org Cc: Mel Gorman mgorman@suse.de Cc: stable@vger.kernel.org 4.20+ Signed-off-by: Yang Shi yang.shi@linux.alibaba.com Signed-off-by: Jan Stancek jstancek@redhat.com --- v3: Adopted fullmm flush suggestion from Will v2: Reworked the commit log per Peter and Will Adopted the suggestion from Peter

mm/mmu_gather.c | 24 +++++++++++++++++++----- 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c index 99740e1..289f8cf 100644 --- a/mm/mmu_gather.c +++ b/mm/mmu_gather.c @@ -245,14 +245,28 @@ void tlb_finish_mmu(struct mmu_gather *tlb, { /* * If there are parallel threads are doing PTE changes on same range - * under non-exclusive lock(e.g., mmap_sem read-side) but defer TLB - * flush by batching, a thread has stable TLB entry can fail to flush - * the TLB by observing pte_none|!pte_dirty, for example so flush TLB - * forcefully if we detect parallel PTE batching threads. + * under non-exclusive lock (e.g., mmap_sem read-side) but defer TLB + * flush by batching, one thread may end up seeing inconsistent PTEs + * and result in having stale TLB entries. So flush TLB forcefully + * if we detect parallel PTE batching threads. + * + * However, some syscalls, e.g. munmap(), may free page tables, this + * needs force flush everything in the given range. Otherwise this + * may result in having stale TLB entries for some architectures, + * e.g. aarch64, that could specify flush what level TLB. */ if (mm_tlb_flush_nested(tlb->mm)) { + /* + * The aarch64 yields better performance with fullmm by + * avoiding multiple CPUs spamming TLBI messages at the + * same time. + * + * On x86 non-fullmm doesn't yield significant difference + * against fullmm. + */ + tlb->fullmm = 1; __tlb_reset_range(tlb); - __tlb_adjust_range(tlb, start, end - start); + tlb->freed_tables = 1; }

tlb_flush_mmu(tlb);