ext/opcache: drop the pre-emptive munmap before the huge page remap - #23574
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ext/opcache: drop the pre-emptive munmap before the huge page remap#23574s2x wants to merge 1 commit into
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create_segments() reserved requested_size + huge_page_size, unmapped the whole reservation, and only then MAP_FIXED-mapped the huge pages at the aligned address inside it. That munmap is unnecessary: MAP_FIXED replaces the overlapped part of the reservation atomically. It also opens a window in which another thread can map something at that address before the remap runs. Keep the reservation, map the huge pages into it, and release only the head and the tail that are left over. Their sizes always add up to exactly huge_page_size, because the reservation is requested_size + huge_page_size and the mapping is requested_size. When the remap fails the whole reservation is released before falling back to normal pages: the kernel may already have discarded the overlapped part, so the reservation cannot be reused. Suggested by Arnaud Le Blanc in phpGH-23554.
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Follow-up to #23554, suggested by @arnaud-lb:
MAP_FIXEDalready replaces the overlapped part of the reservation, so themunmap()before it is unnecessary and only opens a window for another thread to take the address.This keeps the reservation, maps the huge pages into it, and releases only the head and the tail that are left over — they always add up to
huge_page_size. On failure the whole reservation is released, because a failedMAP_FIXEDcan leavea hole in it.
As with my last patch, please check this carefully. This part of the code is still new to me.