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Current clone operation could be non-atomic if the destination of a file is beyond EOF, user could get a file with corrupted (zeroed) data on crash. The problem is about preallocations. If you write some data into a file: [A...B) and XFS decides to preallocate some post-eof blocks, then it can create a delayed allocation reservation: [A.........D) The writeback path tries to convert delayed extents to real ones by allocating blocks. If there aren't enough contiguous free space, we can end up with two extents, the first real and the second still delalloc: [A....C)[C.D) After that, both the in-memory and the on-disk file sizes are still B. If we clone into the range [E...F) from another file: [A....C)[C.D) [E...F) then xfs_reflink_zero_posteof() calls iomap_zero_range() to zero out the range [B, E) beyond EOF and flush it. Since [C, D) is still a delalloc extent, its pagecache will be zeroed and both the in-memory and on-disk size will be updated to D after flushing but before cloning. This is wrong, because the user can see the size change and read the zeroes while the clone operation is ongoing. We need to keep the in-memory and on-disk size before the clone operation starts, so instead of writing zeroes through the page cache for delayed ranges beyond EOF, we convert these ranges to unwritten and invalidate any cached data over that range beyond EOF. Suggested-by: Dave Chinner <david@fromorbit.com> Signed-off-by: Zhang Yi <yi.zhang@huawei.com> Reviewed-by: "Darrick J. Wong" <djwong@kernel.org> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>
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 reStructuredText 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.