58f595c665
Now that GUP no longer requires VM_FAULT_WRITE, break_ksm() is the sole remaining user of VM_FAULT_WRITE. As we also want to stop triggering a fake write fault and instead use FAULT_FLAG_UNSHARE -- similar to GUP-triggered unsharing when taking a R/O pin on a shared anonymous page (including KSM pages), let's stop relying on VM_FAULT_WRITE. Let's rework break_ksm() to not rely on the return value of handle_mm_fault() anymore to figure out whether COW-breaking was successful. Simply perform another follow_page() lookup to verify the result. While this makes break_ksm() slightly less efficient, we can simplify handle_mm_fault() a little and easily switch to FAULT_FLAG_UNSHARE without introducing similar KSM-specific behavior for FAULT_FLAG_UNSHARE. In my setup (AMD Ryzen 9 3900X), running the KSM selftest to test unmerge performance on 2 GiB (taskset 0x8 ./ksm_tests -D -s 2048), this results in a performance degradation of ~4% -- 5% (old: ~5250 MiB/s, new: ~5010 MiB/s). I don't think that we particularly care about that performance drop when unmerging. If it ever turns out to be an actual performance issue, we can think about a better alternative for FAULT_FLAG_UNSHARE -- let's just keep it simple for now. Link: https://lkml.kernel.org/r/20221021101141.84170-3-david@redhat.com Signed-off-by: David Hildenbrand <david@redhat.com> Acked-by: Peter Xu <peterx@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Hugh Dickins <hughd@google.com> Cc: Jason Gunthorpe <jgg@nvidia.com> Cc: John Hubbard <jhubbard@nvidia.com> Cc: Matthew Wilcox (Oracle) <willy@infradead.org> Cc: Shuah Khan <shuah@kernel.org> Cc: Vlastimil Babka <vbabka@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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.