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There's a kernel bug in do_pages_stat(), affecting systems combining 64-bit kernel and 32-bit user space. The function splits the request into chunks of 16 pointers, but forgets the pointers are 32-bit when advancing to the next chunk. Some of the pointers get skipped, and memory after the array is interpreted as pointers. The result is that the produced status of memory pages is mostly bogus. Systems combining 64-bit and 32-bit environments like this might seem rare, but that's not the case - all 32-bit Debian packages are built in a 32-bit chroot on a system with a 64-bit kernel. This is a long-standing kernel bug (since 2010), affecting pretty much all kernels, so it'll take time until all systems get a fixed kernel. Luckily, we can work around the issue by chunking the requests the same way do_pages_stat() does, at least on affected systems. We don't know what kernel a 32-bit build will run on, so all 32-bit builds use chunks of 16 elements (the largest chunk before hitting the issue). 64-bit builds are not affected by this issue, and so could work without the chunking. But chunking has other advantages, so we apply chunking even for 64-bit builds, with chunks of 1024 elements. Reported-by: Christoph Berg <myon@debian.org> Author: Christoph Berg <myon@debian.org> Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com> Discussion: https://postgr.es/m/aEtDozLmtZddARdB@msg.df7cb.de Context: https://marc.info/?l=linux-mm&m=175077821909222&w=2 Backpatch-through: 18 |
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