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9871d09637
Currently xfs_bmap_del_extent_real frees RT extents before updating the bmap btree, while it frees regular blocks after performing the bmap btree update for convoluted historic reasons. Switch to free the RT blocks in the same place as the regular data blocks instead to simply the code and fix a very theoretical bug. A short history of this code researched by Dave Chiner below: The truncate for data device extents was originally a two-phase operation. First it removed the bmapbt record, but because this can free BMBT extents, it can use up all the free space tree reservation space. So the transaction gets rolled to commit the BMBT change and the xfs_bmap_finish() call that frees the data extent runs with a new transaction reservation that allows different free space btrees to be logged without overrun. However, on crash, this could lose the free space because there was nothing to tell recovery about the extents removed from the BMBT, hence EFIs were introduced. They tie the extent free operation to the bmapbt record removal commit for recovery of the second phase of the extent removal process. Then RT extents came along. RT extent freeing does not require a free space btree reservation because the free space metadata is static and transaction size is bound. Hence we don't need to care if the BMBT record removal modifies the per-ag free space trees and we don't need a two-phase extent remove transaction. The only thing we have to care about is not losing space on crash. Hence instead of recording the extent for freeing in the bmap list for xfs_bmap_finish() to process in a new transaction, it simply freed the rtextent directly. So the original code (from 1994) simply replaced the "free AG extent later" queueing with a direct free. This code was originally at the start of xfs_dmap_del_extent(), but the xfs_bmap_add_free() got moved to the end of the function via the "do_fx" flag (the current code logic) in 1997 (commit c4fac74eaa58 in the historic xfs-import tree) because there was a shutdown occurring because of a case where splitting the extent record failed because the BMBT split and the filesystem didn't have enough space for the split to be done. (FWIW, I'm not sure this can happen anymore.) The commit backed out the BMBT change on ENOSPC error, and in doing so I think this actually breaks RT free space tracking. However, it then returns an ENOSPC error, and we have a dirty transaction in the RT case so this will shut down the filesysetm when the transaction is cancelled. Hence the corrupted "bmbt now points at freed rt dev space" condition never make it to disk, but it's still the wrong way to handle the issue. IOWs, this proposed change fixes that "shutdown at ENOSPC on rt devices" situation that was introduced by the above commit back in 1997. Signed-off-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Dave Chinner <dchinner@redhat.com> Reviewed-by: "Darrick J. Wong" <djwong@kernel.org> Signed-off-by: Chandan Babu R <chandanbabu@kernel.org> |
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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 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.