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Darrick J. Wong 227bda0fd0 xfs: restrict when we try to align cow fork delalloc to cowextsz hints [ Upstream commit `288e1f693f` ] xfs/205 produces the following failure when always_cow is enabled: # --- a/tests/xfs/205.out 2024-02-28 16:20:24.437887970 -0800 # +++ b/tests/xfs/205.out.bad 2024-06-03 21:13:40.584000000 -0700 # @@ -1,4 +1,5 @@ # QA output created by 205 # * one file # + !!! disk full (expected) # * one file, a few bytes at a time # *** done This is the result of overly aggressive attempts to align cow fork delalloc reservations to the CoW extent size hint. Looking at the trace data, we're trying to append a single fsblock to the "fred" file. Trying to create a speculative post-eof reservation fails because there's not enough space. We then set @prealloc_blocks to zero and try again, but the cowextsz alignment code triggers, which expands our request for a 1-fsblock reservation into a 39-block reservation. There's not enough space for that, so the whole write fails with ENOSPC even though there's sufficient space in the filesystem to allocate the single block that we need to land the write. There are two things wrong here -- first, we shouldn't be attempting speculative preallocations beyond what was requested when we're low on space. Second, if we've already computed a posteof preallocation, we shouldn't bother trying to align that to the cowextsize hint. Fix both of these problems by adding a flag that only enables the expansion of the delalloc reservation to the cowextsize if we're doing a non-extending write, and only if we're not doing an ENOSPC retry. This requires us to move the ENOSPC retry logic to xfs_bmapi_reserve_delalloc. I probably should have caught this six years ago when `6ca30729c2` was being reviewed, but oh well. Update the comments to reflect what the code does now. Fixes: `6ca30729c2` ("xfs: bmap code cleanup") Signed-off-by: Darrick J. Wong <djwong@kernel.org> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Chandan Babu R <chandanbabu@kernel.org> Signed-off-by: Leah Rumancik <leah.rumancik@gmail.com> Acked-by: "Darrick J. Wong" <djwong@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>		History 2025-05-09 09:41:38 +02:00
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README

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.