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Since i_version is mostly treated as an opaque value, we can exploit that fact to avoid incrementing it when no one is watching. With that change, we can avoid incrementing the counter on writes, unless someone has queried for it since it was last incremented. If the a/c/mtime don't change, and the i_version hasn't changed, then there's no need to dirty the inode metadata on a write. Convert the i_version counter to an atomic64_t, and use the lowest order bit to hold a flag that will tell whether anyone has queried the value since it was last incremented. When we go to maybe increment it, we fetch the value and check the flag bit. If it's clear then we don't need to do anything if the update isn't being forced. If we do need to update, then we increment the counter by 2, and clear the flag bit, and then use a CAS op to swap it into place. If that works, we return true. If it doesn't then do it again with the value that we fetch from the CAS operation. On the query side, if the flag is already set, then we just shift the value down by 1 bit and return it. Otherwise, we set the flag in our on-stack value and again use cmpxchg to swap it into place if it hasn't changed. If it has, then we use the value from the cmpxchg as the new "old" value and try again. This method allows us to avoid incrementing the counter on writes (and dirtying the metadata) under typical workloads. We only need to increment if it has been queried since it was last changed. Signed-off-by: Jeff Layton <jlayton@redhat.com> Reviewed-by: Jan Kara <jack@suse.cz> Acked-by: Dave Chinner <dchinner@redhat.com> Tested-by: Krzysztof Kozlowski <krzk@kernel.org> |
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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.