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[2003:cb:c744:ac00:ef5c:b66d:1075:254a]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-42f57e13332sm2458795e9.38.2024.09.26.08.25.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 26 Sep 2024 08:25:08 -0700 (PDT) Message-ID: <72ea5f41-6186-4383-90e4-cdd04fac6a3c@redhat.com> Date: Thu, 26 Sep 2024 17:25:07 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [syzbot] [mm?] WARNING in copy_huge_pmd To: Peter Xu Cc: syzbot , akpm@linux-foundation.org, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, jgg@ziepe.ca, leitao@debian.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mingo@redhat.com, rppt@kernel.org, syzkaller-bugs@googlegroups.com, tglx@linutronix.de, x86@kernel.org References: <66f15c8d.050a0220.c23dd.000f.GAE@google.com> <4f96130c-12b7-4afa-ada3-bec354576112@redhat.com> <60e29e62-4864-4393-b899-01489ee73b91@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 26.09.24 15:39, Peter Xu wrote: > On Thu, Sep 26, 2024 at 12:48:19PM +0200, David Hildenbrand wrote: >> On 25.09.24 18:59, Peter Xu wrote: >>> On Tue, Sep 24, 2024 at 04:45:00PM +0200, David Hildenbrand wrote: >>>> On 23.09.24 14:18, syzbot wrote: >>>>> Hello, >>>>> >>>>> syzbot found the following issue on: >>>>> >>>>> HEAD commit: 88264981f208 Merge tag 'sched_ext-for-6.12' of git://git.k.. >>>>> git tree: upstream >>>>> console+strace: https://syzkaller.appspot.com/x/log.txt?x=16c36c27980000 >>>>> kernel config: https://syzkaller.appspot.com/x/.config?x=e851828834875d6f >>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=bf2c35fa302ebe3c7471 >>>>> compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40 >>>>> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=12773080580000 >>>>> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16ed5e9f980000 >>>>> >>>>> Downloadable assets: >>>>> disk image: https://storage.googleapis.com/syzbot-assets/0e011ac37c93/disk-88264981.raw.xz >>>>> vmlinux: https://storage.googleapis.com/syzbot-assets/f5c65577e19e/vmlinux-88264981.xz >>>>> kernel image: https://storage.googleapis.com/syzbot-assets/984d963c8ea1/bzImage-88264981.xz >>>>> >>>>> The issue was bisected to: >>>>> >>>>> commit 75182022a0439788415b2dd1db3086e07aa506f7 >>>>> Author: Peter Xu >>>>> Date: Mon Aug 26 20:43:51 2024 +0000 >>>>> >>>>> mm/x86: support large pfn mappings >>>>> >>>>> bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=17df9c27980000 >>>>> final oops: https://syzkaller.appspot.com/x/report.txt?x=143f9c27980000 >>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=103f9c27980000 >>>>> >>>>> IMPORTANT: if you fix the issue, please add the following tag to the commit: >>>>> Reported-by: syzbot+bf2c35fa302ebe3c7471@syzkaller.appspotmail.com >>>>> Fixes: 75182022a043 ("mm/x86: support large pfn mappings") >>>>> >>>>> ------------[ cut here ]------------ >>>>> WARNING: CPU: 1 PID: 5508 at mm/huge_memory.c:1602 copy_huge_pmd+0x102c/0x1c60 mm/huge_memory.c:1602 >>>> >>>> This is the >>>> >>>> VM_WARN_ON_ONCE(is_cow_mapping(src_vma->vm_flags) && pmd_write(pmd)) >>>> >>>> So we have a special-marked PMD in a COW mapping. >>>> >>>> The reproducer seems to involve fuse, but not sure if that makes a >>>> difference here. >>> >>> That chunk of code seems to be there only making sure the test won't get >>> blocked due to any fused based fs being stuck, via writting to the "abort" >>> file: >>> >>> snprintf(abort, sizeof(abort), "/sys/fs/fuse/connections/%s/abort", >>> ent->d_name); >>> int fd = open(abort, O_WRONLY); >>> if (fd == -1) { >>> continue; >>> } >>> if (write(fd, abort, 1) < 0) { >>> } >>> close(fd); >>> >>> So far looks not relevant to this issue indeed. >>> >>> Unfortunately I cannot reproduce it even with the reproducer. So this one >>> is a bit tricky.. >>> >>> What confuses me yet is how that special bit is set, if it's only used so >>> far with vfio-pci, and this test doesn't seem to have it involved. >>> >>> The test keeps invoking processes, then threads, doing concurrent accesses >>> over a few stuff (madvise, mremap, migrate_pages, munmap, etc.) on the >>> pre-mapped areas, but none of them seem to create new memory that can >>> provide hint on how special bit can start to occur. >>> >>> I wonder if some of these operations can race in a way that mm can wrongly >>> create the special bit (alone with it being writable).. and then it could >>> be a historical bug, only captured by this patchset due to the newly added >>> WARN_ON_ONCE somehow, then it could mean that it's not the WRITE bit that >>> is not intended, but the SPECIAL bit altogether. >> >> I assume you are missing a check for present/non-swap pmds. Assume you have >> a migration entry and end up using the special bit -- which is perfectly >> fine -- your code would assume it's a present PMD with the special bit set. >> >> Maybe for the time being something like: >> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index 0580ac9e47b9..e55efcad1e6c 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -1586,7 +1586,7 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct >> mm_struct *src_mm, >> int ret = -ENOMEM; >> >> pmd = pmdp_get_lockless(src_pmd); >> - if (unlikely(pmd_special(pmd))) { >> + if (unlikely(pmd_present(pmd) && pmd_special(pmd))) { >> dst_ptl = pmd_lock(dst_mm, dst_pmd); >> src_ptl = pmd_lockptr(src_mm, src_pmd); >> spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING); > > Good catch! > > I definitely overlooked it, and I did check the config has THP_MIGRATION > set indeed. So it's very possible relevant. > > Do you want to send a formal patch? You can also push a branch with "#syz > test", looks like syzbot can constantly reproduce. Yes, let me send out a patch real quick. -- Cheers, David / dhildenb