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User-Agent: Alpine 2.02 (LRH 1266 2009-07-14) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Message-ID-Hash: TZ6JWIUA37RBDOYPRDDOBH6IGOJFFZYL X-Message-ID-Hash: TZ6JWIUA37RBDOYPRDDOBH6IGOJFFZYL X-MailFrom: mpatocka@redhat.com X-Mailman-Rule-Hits: nonmember-moderation X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation CC: Peter Xu , Jann Horn , Christoph Hellwig , Oleg Nesterov , Kirill Shutemov , Jan Kara , Andrea Arcangeli , Matthew Wilcox , Andrew Morton , Linux-MM , Linux Kernel Mailing List , linux-nvdimm X-Mailman-Version: 3.1.1 Precedence: list List-Id: "Linux-nvdimm developer list." Archived-At: List-Archive: List-Help: List-Post: List-Subscribe: List-Unsubscribe: Content-Type: TEXT/PLAIN; charset="us-ascii" Content-Transfer-Encoding: 7bit On Thu, 3 Sep 2020, Linus Torvalds wrote: > On Thu, Sep 3, 2020 at 12:24 PM Mikulas Patocka wrote: > > > > There's a bug when you run strace from dax-based filesystem. > > > > -- create real or emulated persistent memory device (/dev/pmem0) > > mkfs.ext2 /dev/pmem0 > > -- mount it > > mount -t ext2 -o dax /dev/pmem0 /mnt/test > > -- copy the system to it (well, you can copy just a few files that are > > needed for running strace and ls) > > cp -ax / /mnt/test > > -- bind the system directories > > mount --bind /dev /mnt/test/dev > > mount --bind /proc /mnt/test/proc > > mount --bind /sys /mnt/test/sys > > -- run strace on the ls command > > chroot /mnt/test/ strace /bin/ls > > > > You get this warning and ls is killed with SIGSEGV. > > > > I bisected the problem and it is caused by the commit > > 17839856fd588f4ab6b789f482ed3ffd7c403e1f (gup: document and work around > > "COW can break either way" issue). When I revert the patch (on the kernel > > 5.9-rc3), the bug goes away. > > Funky. I really don't see how it could cause that, but we have the > UDDF issue too, so I'm guessing I will have to fix it the radical way > with Peter Xu's series based on my "rip out COW special cases" patch. > > Or maybe I'm just using that as an excuse for really wanting to apply > that series.. Because we can't just revert that GUP commit due to > security concerns. > > > [ 84.191504] WARNING: CPU: 6 PID: 1350 at mm/memory.c:2486 wp_page_copy.cold+0xdb/0xf6 > > I'm assuming this is the WARN_ON_ONCE(1) on line 2482, and you have > some extra debug patch that causes that line to be off by 4? Because > at least for me, line 2486 is actually an empty line in v5.9-rc3. Yes, that's it. I added a few printk to look at the control flow. > That said, I really think this is a pre-existing race, and all the > "COW can break either way" patch does is change the timing (presumably > due to the actual pattern of actually doing the COW changing). > > See commit c3e5ea6ee574 ("mm: avoid data corruption on CoW fault into > PFN-mapped VMA") for background. > > Mikulas, can you check that everything works ok for that case if you > apply Peter's series? See > > https://lore.kernel.org/lkml/20200821234958.7896-1-peterx@redhat.com/ I applied these four patches and strace works well. There is no longer any warning or crash. Mikulas > or if you have 'b4' installed, use > > b4 am 20200821234958.7896-1-peterx@redhat.com > > to get the series.. > > Linus > _______________________________________________ Linux-nvdimm mailing list -- linux-nvdimm@lists.01.org To unsubscribe send an email to linux-nvdimm-leave@lists.01.org