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[2003:cb:c744:ac00:ef5c:b66d:1075:254a]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-37ccca61e15sm2657340f8f.88.2024.09.26.03.48.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 26 Sep 2024 03:48:20 -0700 (PDT) Message-ID: <60e29e62-4864-4393-b899-01489ee73b91@redhat.com> Date: Thu, 26 Sep 2024 12:48:19 +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> 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 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); -- Cheers, David / dhildenb