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9118264507
btrfs_space_info_add_old_bytes is used when adding the extra space from an existing reservation back into the space_info to be used by any waiting tickets. In order to keep us from overcommitting we check to make sure that we can still use this space for our reserve ticket, and if we cannot we'll simply subtract it from space_info->bytes_may_use. However this is problematic, because it assumes that only changes to bytes_may_use would affect our ability to make reservations. Any changes to bytes_reserved would be missed. If we were unable to make a reservation prior because of reserved space, but that reserved space was free'd due to unlink or truncate and we were allowed to immediately reclaim that metadata space we would still ENOSPC. Consider the example where we create a file with a bunch of extents, using up 2MiB of actual space for the new tree blocks. Then we try to make a reservation of 2MiB but we do not have enough space to make this reservation. The iput() occurs in another thread and we remove this space, and since we did not write the blocks we simply do space_info->bytes_reserved -= 2MiB. We would never see this because we do not check our space info used, we just try to re-use the freed reservations. To fix this problem, and to greatly simplify the wakeup code, do away with this partial refilling nonsense. Use btrfs_space_info_add_old_bytes to subtract the reservation from space_info->bytes_may_use, and then check the ticket against the total used of the space_info the same way we do with the initial reservation attempt. This keeps the reservation logic consistent and solves the problem of early ENOSPC in the case that we free up space in places other than bytes_may_use and bytes_pinned. Thanks, Signed-off-by: Josef Bacik <josef@toxicpanda.com> Signed-off-by: David Sterba <dsterba@suse.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.