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posix_fallocate() will allocate space in an NFS file by considering the last byte of every 4K block. If it is before EOF, it will read the byte and if it is zero, a zero is written out. If it is after EOF, the zero is unconditionally written. For the blocks beyond EOF, if NFS believes its cache is valid, it will expand these writes to write full pages, and then will merge the pages. This results if (typically) 1MB writes. If NFS believes its cache is not valid (particularly if NFS_INO_INVALID_DATA or NFS_INO_REVAL_PAGECACHE are set - see nfs_write_pageuptodate()), it will send the individual 1-byte writes. This results in (typically) 256 times as many RPC requests, and can be substantially slower. Currently nfs_revalidate_mapping() is only used when reading a file or mmapping a file, as these are times when the content needs to be up-to-date. Writes don't generally need the cache to be up-to-date, but writes beyond EOF can benefit, particularly in the posix_fallocate() case. So this patch calls nfs_revalidate_mapping() when writing beyond EOF - i.e. when there is a gap between the end of the file and the start of the write. If the cache is thought to be out of date (as happens after taking a file lock), this will cause a GETATTR, and the two flags mentioned above will be cleared. With this, posix_fallocate() on a newly locked file does not generate excessive tiny writes. Signed-off-by: NeilBrown <neilb@suse.com> Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.