When forking a child process, parent write-protects an anonymous page and COW-shares it with the child being forked using copy_present_pte(). Parent's TLB is flushed right before we drop the parent's mmap_lock in dup_mmap(). If we get a write-fault before that TLB flush in the parent, and we end up replacing that anonymous page in the parent process in do_wp_page() (because, COW-shared with the child), this might lead to some stale writable TLB entries targeting the wrong (old) page. Similar issue happened in the past with userfaultfd (see flush_tlb_page() call inside do_wp_page()). Lock VMAs of the parent process when forking a child, which prevents concurrent page faults during fork operation and avoids this issue. This fix can potentially regress some fork-heavy workloads. Kernel build time did not show noticeable regression on a 56-core machine while a stress test mapping 10000 VMAs and forking 5000 times in a tight loop shows ~5% regression. If such fork time regression is unacceptable, disabling CONFIG_PER_VMA_LOCK should restore its performance. Further optimizations are possible if this regression proves to be problematic. Suggested-by: David Hildenbrand <david@xxxxxxxxxx> Reported-by: Jiri Slaby <jirislaby@xxxxxxxxxx> Closes: https://lore.kernel.org/all/dbdef34c-3a07-5951-e1ae-e9c6e3cdf51b@xxxxxxxxxx/ Reported-by: Holger Hoffstätte <holger@xxxxxxxxxxxxxxxxxxxxxx> Closes: https://lore.kernel.org/all/b198d649-f4bf-b971-31d0-e8433ec2a34c@xxxxxxxxxxxxxxxxxxxxxx/ Reported-by: Jacob Young <jacobly.alt@xxxxxxxxx> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217624 Fixes: 0bff0aaea03e ("x86/mm: try VMA lock-based page fault handling first") Cc: stable@xxxxxxxxxxxxxxx Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> --- kernel/fork.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/kernel/fork.c b/kernel/fork.c index b85814e614a5..403bc2b72301 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -658,6 +658,12 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, retval = -EINTR; goto fail_uprobe_end; } +#ifdef CONFIG_PER_VMA_LOCK + /* Disallow any page faults before calling flush_cache_dup_mm */ + for_each_vma(old_vmi, mpnt) + vma_start_write(mpnt); + vma_iter_init(&old_vmi, oldmm, 0); +#endif flush_cache_dup_mm(oldmm); uprobe_dup_mmap(oldmm, mm); /* -- 2.41.0.255.g8b1d071c50-goog
