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The writeback delalloc conversion code is racy with respect to changes in the currently cached file mapping outside of the current page. This is because the ilock is cycled between the time the caller originally looked up the mapping and across each real allocation of the provided file range. This code has collected various hacks over the years to help combat the symptoms of these races (i.e., truncate race detection, allocation into hole detection, etc.), but none address the fundamental problem that the imap may not be valid at allocation time. Rather than continue to use race detection hacks, update writeback delalloc conversion to a model that explicitly converts the delalloc extent backing the current file offset being processed. The current file offset is the only block we can trust to remain once the ilock is dropped because any operation that can remove the block (truncate, hole punch, etc.) must flush and discard pagecache pages first. Modify xfs_iomap_write_allocate() to use the xfs_bmapi_delalloc() mechanism to request allocation of the entire delalloc extent backing the current offset instead of assuming the extent passed by the caller is unchanged. Record the range specified by the caller and apply it to the resulting allocated extent so previous checks by the caller for COW fork overlap are not lost. Finally, overload the bmapi delalloc flag with the range reval flag behavior since this is the only use case for both. This ensures that writeback always picks up the correct and current extent associated with the page, regardless of races with other extent modifying operations. If operating on a data fork and the COW overlap state has changed since the ilock was cycled, the caller revalidates against the COW fork sequence number before using the imap for the next block. Signed-off-by: Brian Foster <bfoster@redhat.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
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.