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[2003:cb:c746:1b00:fd9e:c26c:c552:1de7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-434aa74fec9sm222965495e9.6.2024.12.03.07.00.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 03 Dec 2024 07:00:24 -0800 (PST) Message-ID: Date: Tue, 3 Dec 2024 16:00:22 +0100 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 0/7] hugetlbfs memory HW error fixes To: William Roche , kvm@vger.kernel.org, qemu-devel@nongnu.org, qemu-arm@nongnu.org Cc: peterx@redhat.com, pbonzini@redhat.com, richard.henderson@linaro.org, philmd@linaro.org, peter.maydell@linaro.org, mtosatti@redhat.com, imammedo@redhat.com, eduardo@habkost.net, marcel.apfelbaum@gmail.com, wangyanan55@huawei.com, zhao1.liu@intel.com, joao.m.martins@oracle.com References: <20241125142718.3373203-1-william.roche@oracle.com> <48b09647-d2ba-43e5-8e73-16fb4ace6da5@oracle.com> <874e2625-b5e7-4247-994a-9b341abbdceb@redhat.com> <753a033c-7341-4a3a-8546-c31a50d35aff@redhat.com> <31693f78-dbef-4465-9c8e-a68a25cb4af2@oracle.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: 8bit On 03.12.24 15:39, William Roche wrote: > On 12/3/24 15:08, David Hildenbrand wrote: >> [...] >> >> Let me take a look at your tool below if I can find an explanation of >> what is happening, because it's weird :) >> >> [...] >> >>> >>> At the end of this email, I included the source code of a simplistic >>> test case that shows that the page is replaced in the case of standard >>> page size. >>> >>> The idea of this test is simple: >>> >>> 1/ Create a local FILE with: >>> # dd if=/dev/zero of=./FILE bs=4k count=2 >>> 2+0 records in >>> 2+0 records out >>> 8192 bytes (8.2 kB, 8.0 KiB) copied, 0.000337674 s, 24.3 MB/s >>> >>> 2/ As root run: >>> # ./poisonedShared4k >>> Mapping 8192 bytes from file FILE >>> Reading and writing the first 2 pages content: >>> Read: Read: Wrote: Initial mem page 0 >>> Wrote: Initial mem page 1 >>> Data pages at 0x7f71a19d6000  physically 0x124fb0000 >>> Data pages at 0x7f71a19d7000  physically 0x128ce4000 >>> Poisoning 4k at 0x7f71a19d6000 >>> Signal 7 received >>>     code 4        Signal code >>>     addr 0x7f71a19d6000    Memory location >>>     si_addr_lsb 12 >>> siglongjmp used >>> Remapping the poisoned page >>> Reading and writing the first 2 pages content: >>> Read: Read: Initial mem page 1 >>> Wrote: Rewrite mem page 0 >>> Wrote: Rewrite mem page 1 >>> Data pages at 0x7f71a19d6000  physically 0x10c367000 >>> Data pages at 0x7f71a19d7000  physically 0x128ce4000 >>> >>> >>>    --- >>> >>> As we can see, this process: >>> - maps the FILE, >>> - tries to read and write the beginning of the first 2 pages >>> - gives their physical addresses >>> - poison the first page with a madvise(MADV_HWPOISON) call >>> - shows the SIGBUS signal received and recovers from it >>> - simply remaps the same page from the file >>> - tries again to read and write the beginning of the first 2 pages >>> - gives their physical addresses >>> >> >> >> Turns out the code will try to truncate the pagecache page using >> mapping->a_ops->error_remove_folio(). >> >> That, however, is only implemented on *some* filesystems. >> >> Most prominently, it is not implemented on shmem as well. >> >> >> So if you run your test with shmem (e.g., /tmp/FILE), it doesn't work. > > Correct, on tmpfs the test case fails to continue to use the memory area > and gets a SIGBUS. And it works with xfs. > > > >> >> Using fallocate+MADV_DONTNEED seems to work on shmem. >> > > Our new Qemu code is testing first the fallocate+MADV_DONTNEED procedure > for standard sized pages (in ram_block_discard_range()) and only folds > back to the mmap() use if it fails. So maybe my proposal to implement: > > + /* > + * Fold back to using mmap(), but it should not > repair a > + * shared file memory region. In this case we fail. > + */ > + if (block->fd >= 0 && qemu_ram_is_shared(block)) { > + error_report("Shared memory poison recovery > failure addr: " > + RAM_ADDR_FMT "@" RAM_ADDR_FMT "", > + length, addr); > + exit(1); > + } > > Could be the right choice. Right. But then, what about a mmap(MAP_PRIVATE, shmem), where the pagecache page is poisoned and needs an explicit fallocate? :) It's all tricky. I wonder if we should just say "if it's backed by a file, and we cannot discard, then mmap() can't fix it reliably". if (block->fd >= 0) { ... } After all, we don't even expect the fallocate/MADV_DONTNEED to ever fail :) So I was also wondering if we could get rid of the mmap(MAP_FIXED) completely ... but who knows what older Linux kernels do. -- Cheers, David / dhildenb