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When doing file lookups and checking for permissions, we end up in xfs_get_acl() to see if there are any ACLs on the inode. This requires and xattr lookup, and to do that we have to supply a buffer large enough to hold an maximum sized xattr. On workloads were we are accessing a wide range of cache cold files under memory pressure (e.g. NFS fileservers) we end up spending a lot of time allocating the buffer. The buffer is 64k in length, so is a contiguous multi-page allocation, and if that then fails we fall back to vmalloc(). Hence the allocation here is /expensive/ when we are looking up hundreds of thousands of files a second. Initial numbers from a bpf trace show average time in xfs_get_acl() is ~32us, with ~19us of that in the memory allocation. Note these are average times, so there are going to be affected by the worst case allocations more than the common fast case... To avoid this, we could just do a "null" lookup to see if the ACL xattr exists and then only do the allocation if it exists. This, however, optimises the path for the "no ACL present" case at the expense of the "acl present" case. i.e. we can halve the time in xfs_get_acl() for the no acl case (i.e down to ~10-15us), but that then increases the ACL case by 30% (i.e. up to 40-45us). To solve this and speed up both cases, drive the xattr buffer allocation into the attribute code once we know what the actual xattr length is. For the no-xattr case, we avoid the allocation completely, speeding up that case. For the common ACL case, we'll end up with a fast heap allocation (because it'll be smaller than a page), and only for the rarer "we have a remote xattr" will we have a multi-page allocation occur. Hence the common ACL case will be much faster, too. Signed-off-by: Dave Chinner <dchinner@redhat.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> |
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| fs | ||
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| kernel | ||
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| 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.