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It is not immediately obvious, but we can move the folio->_nr_pages_mapped update out of the loop and reduce the number of atomic ops without affecting the stats. The important point to realize is that only removing the last PMD mapping will result in _nr_pages_mapped going below ENTIRELY_MAPPED, not the individual atomic_inc_return_relaxed() calls. Concurrent races with removal of PMD mappings should be handled as expected, just like when we would have such races right now on a single mapcount update. In a simple munmap() microbenchmark [1] on 1 GiB of memory backed by the same PTE-mapped folio size (only mapped by a single process such that they will get completely unmapped), this change results in a speedup (positive is good) per folio size on a x86-64 Intel machine of roughly (a bit of noise expected): * 16 KiB: +10% * 32 KiB: +15% * 64 KiB: +17% * 128 KiB: +21% * 256 KiB: +22% * 512 KiB: +22% * 1024 KiB: +23% * 2048 KiB: +27% [1] https://gitlab.com/davidhildenbrand/scratchspace/-/blob/main/pte-mapped-folio-benchmarks.c Link: https://lkml.kernel.org/r/20240807115515.1640951-1-david@redhat.com Signed-off-by: David Hildenbrand <david@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.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.