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xfs_bmap_btalloc is given a range of file offset blocks that must be allocated to some data/attr/cow fork. If the fork has an extent size hint associated with it, the request will be enlarged on both ends to try to satisfy the alignment hint. If free space is fragmentated, sometimes we can allocate some blocks but not enough to fulfill any of the requested range. Since bmapi_allocate always trims the new extent mapping to match the originally requested range, this results in bmapi_write returning zero and no mapping. The consequences of this vary -- buffered writes will simply re-call bmapi_write until it can satisfy at least one block from the original request. Direct IO overwrites notice nmaps == 0 and return -ENOSPC through the dio mechanism out to userspace with the weird result that writes fail even when we have enough space because the ENOSPC return overrides any partial write status. For direct CoW writes the situation was disastrous because nobody notices us returning an invalid zero-length wrong-offset mapping to iomap and the write goes off into space. Therefore, if free space is so fragmented that we managed to allocate some space but not enough to map into even a single block of the original allocation request range, we should break the alignment hint in order to guarantee at least some forward progress for the direct write. If we return a short allocation to iomap_apply it'll call back about the remaining blocks. Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> Reviewed-by: Christoph Hellwig <hch@lst.de> |
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| certs | ||
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| Documentation | ||
| drivers | ||
| firmware | ||
| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
| mm | ||
| net | ||
| samples | ||
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| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
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| COPYING | ||
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| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
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.