mainlining shenanigans
54752de928
The historic requirement for XFS to invalidate cached pages on direct IO reads has been lost in the twisty pages of history - it was inherited from Irix, which implemented page cache invalidation on read as a method of working around problems synchronising page cache state with uncached IO. XFS has carried this ever since. In the initial linux ports it was necessary to get mmap and DIO to play "ok" together and not immediately corrupt data. This was the state of play until the linux kernel had infrastructure to track unwritten extents and synchronise page faults with allocations and unwritten extent conversions (->page_mkwrite infrastructure). IOws, the page cache invalidation on DIO read was necessary to prevent trivial data corruptions. This didn't solve all the problems, though. There were peformance problems if we didn't invalidate the entire page cache over the file on read - we couldn't easily determine if the cached pages were over the range of the IO, and invalidation required taking a serialising lock (i_mutex) on the inode. This serialising lock was an issue for XFS, as it was the only exclusive lock in the direct Io read path. Hence if there were any cached pages, we'd just invalidate the entire file in one go so that subsequent IOs didn't need to take the serialising lock. This was a problem that prevented ranged invalidation from being particularly useful for avoiding the remaining coherency issues. This was solved with the conversion of i_mutex to i_rwsem and the conversion of the XFS inode IO lock to use i_rwsem. Hence we could now just do ranged invalidation and the performance problem went away. However, page cache invalidation was still needed to serialise sub-page/sub-block zeroing via direct IO against buffered IO because bufferhead state attached to the cached page could get out of whack when direct IOs were issued. We've removed bufferheads from the XFS code, and we don't carry any extent state on the cached pages anymore, and so this problem has gone away, too. IOWs, it would appear that we don't have any good reason to be invalidating the page cache on DIO reads anymore. Hence remove the invalidation on read because it is unnecessary overhead, not needed to maintain coherency between mmap/buffered access and direct IO anymore, and prevents anyone from using direct IO reads from intentionally invalidating the page cache of a file. Signed-off-by: Dave Chinner <dchinner@redhat.com> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com> Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org> Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> |
||
|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| 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.