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In DAX there are two separate places where the 2MiB range of a PMD is defined. The first is in the page tables, where a PMD mapping inserted for a given address spans from (vmf->address & PMD_MASK) to ((vmf->address & PMD_MASK) + PMD_SIZE - 1). That is, from the 2MiB boundary below the address to the 2MiB boundary above the address. So, for example, a fault at address 3MiB (0x30 0000) falls within the PMD that ranges from 2MiB (0x20 0000) to 4MiB (0x40 0000). The second PMD range is in the mapping->page_tree, where a given file offset is covered by a radix tree entry that spans from one 2MiB aligned file offset to another 2MiB aligned file offset. So, for example, the file offset for 3MiB (pgoff 768) falls within the PMD range for the order 9 radix tree entry that ranges from 2MiB (pgoff 512) to 4MiB (pgoff 1024). This system works so long as the addresses and file offsets for a given mapping both have the same offsets relative to the start of each PMD. Consider the case where the starting address for a given file isn't 2MiB aligned - say our faulting address is 3 MiB (0x30 0000), but that corresponds to the beginning of our file (pgoff 0). Now all the PMDs in the mapping are misaligned so that the 2MiB range defined in the page tables never matches up with the 2MiB range defined in the radix tree. The current code notices this case for DAX faults to storage with the following test in dax_pmd_insert_mapping(): if (pfn_t_to_pfn(pfn) & PG_PMD_COLOUR) goto unlock_fallback; This test makes sure that the pfn we get from the driver is 2MiB aligned, and relies on the assumption that the 2MiB alignment of the pfn we get back from the driver matches the 2MiB alignment of the faulting address. However, faults to holes were not checked and we could hit the problem described above. This was reported in response to the NVML nvml/src/test/pmempool_sync TEST5: $ cd nvml/src/test/pmempool_sync $ make TEST5 You can grab NVML here: https://github.com/pmem/nvml/ The dmesg warning you see when you hit this error is: WARNING: CPU: 13 PID: 2900 at fs/dax.c:641 dax_insert_mapping_entry+0x2df/0x310 Where we notice in dax_insert_mapping_entry() that the radix tree entry we are about to replace doesn't match the locked entry that we had previously inserted into the tree. This happens because the initial insertion was done in grab_mapping_entry() using a pgoff calculated from the faulting address (vmf->address), and the replacement in dax_pmd_load_hole() => dax_insert_mapping_entry() is done using vmf->pgoff. In our failure case those two page offsets (one calculated from vmf->address, one using vmf->pgoff) point to different order 9 radix tree entries. This failure case can result in a deadlock because the radix tree unlock also happens on the pgoff calculated from vmf->address. This means that the locked radix tree entry that we swapped in to the tree in dax_insert_mapping_entry() using vmf->pgoff is never unlocked, so all future faults to that 2MiB range will block forever. Fix this by validating that the faulting address's PMD offset matches the PMD offset from the start of the file. This check is done at the very beginning of the fault and covers faults that would have mapped to storage as well as faults to holes. I left the COLOUR check in dax_pmd_insert_mapping() in place in case we ever hit the insanity condition where the alignment of the pfn we get from the driver doesn't match the alignment of the userspace address. Link: http://lkml.kernel.org/r/20170822222436.18926-1-ross.zwisler@linux.intel.com Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com> Reported-by: "Slusarz, Marcin" <marcin.slusarz@intel.com> Reviewed-by: Jan Kara <jack@suse.cz> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: Christoph Hellwig <hch@lst.de> Cc: Dan Williams <dan.j.williams@intel.com> Cc: Dave Chinner <david@fromorbit.com> Cc: Matthew Wilcox <mawilcox@microsoft.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.