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dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=R63vfxOS; spf=none (imf04.hostedemail.com: domain of peterx@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=peterx@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-HE-Tag: 1632513577-872455 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Sep 24, 2021 at 10:21:30AM -0700, Axel Rasmussen wrote: > On Thu, Sep 23, 2021 at 4:25 PM Peter Xu wrote: > > > > In RHEL's gating selftests we've encountered memory corruption in the uffd > > event test even with upstream kernel: > > > > # ./userfaultfd anon 128 4 > > nr_pages: 32768, nr_pages_per_cpu: 32768 > > bounces: 3, mode: rnd racing read, userfaults: 6240 missing (6240) 14729 wp (14729) > > bounces: 2, mode: racing read, userfaults: 1444 missing (1444) 28877 wp (28877) > > bounces: 1, mode: rnd read, userfaults: 6055 missing (6055) 14699 wp (14699) > > bounces: 0, mode: read, userfaults: 82 missing (82) 25196 wp (25196) > > testing uffd-wp with pagemap (pgsize=4096): done > > testing uffd-wp with pagemap (pgsize=2097152): done > > testing events (fork, remap, remove): ERROR: nr 32427 memory corruption 0 1 (errno=0, line=963) > > ERROR: faulting process failed (errno=0, line=1117) > > > > It can be easily reproduced when global thp enabled, which is the default for > > RHEL. > > > > It's also known as a side effect of commit 0db282ba2c12 ("selftest: use mmap > > instead of posix_memalign to allocate memory", 2021-07-23), which is imho right > > itself on using mmap() to make sure the addresses will be untagged even on arm. > > > > The problem is, for each test we allocate buffers using two allocate_area() > > calls. We assumed these two buffers won't affect each other, however they > > could, because mmap() could have found that the two buffers are near each other > > and having the same VMA flags, so they got merged into one VMA. > > > > It won't be a big problem if thp is not enabled, but when thp is agressively > > enabled it means when initializing the src buffer it could accidentally setup > > part of the dest buffer too when there's a shared THP that overlaps the two > > regions. Then some of the dest buffer won't be able to be trapped by > > userfaultfd missing mode, then it'll cause memory corruption as described. > > > > To fix it, do release_pages() after initializing the src buffer. > > But, if I understand correctly, release_pages() will just free the > physical pages, but not touch the VMA(s). So, with the right > max_ptes_none setting, why couldn't khugepaged just decide to > re-collapse (with zero pages) immediately after we release the pages, > causing the same problem? It seems to me this change just > significantly narrows the race window (which explains why we see less > of the issue), but doesn't fix it fundamentally. Did you mean you can reproduce the issue even with this patch? It is a good point anyway, indeed I don't see anything stops it from happening. I wanted to prepare a v2 by releasing the pages after uffdio registration where we'll do the vma split, but it won't simply work because release_pages() will cause the process to hang death since that test registers with EVENT_REMOVE, and release_pages() upon the thp will trigger synchronous EVENT_REMOVE which cannot be handled by anyone. Another solution is to map some PROT_NONE regions between the buffers, to make sure they won't share a VMA. I'll need to think more about which is better.. -- Peter Xu