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 Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> --- mm/rmap.c | 27 +++++++++++++-------------- 1 file changed, 13 insertions(+), 14 deletions(-) diff --git a/mm/rmap.c b/mm/rmap.c index 901950200957..a6b9cd0b2b18 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1158,7 +1158,7 @@ static __always_inline unsigned int __folio_add_rmap(struct folio *folio, { atomic_t *mapped = &folio->_nr_pages_mapped; const int orig_nr_pages = nr_pages; - int first, nr = 0; + int first = 0, nr = 0; __folio_rmap_sanity_checks(folio, page, nr_pages, level); @@ -1170,13 +1170,13 @@ static __always_inline unsigned int __folio_add_rmap(struct folio *folio, } do { - first = atomic_inc_and_test(&page->_mapcount); - if (first) { - first = atomic_inc_return_relaxed(mapped); - if (first < ENTIRELY_MAPPED) - nr++; - } + first += atomic_inc_and_test(&page->_mapcount); } while (page++, --nr_pages > 0); + + if (first && + atomic_add_return_relaxed(first, mapped) < ENTIRELY_MAPPED) + nr = first; + atomic_add(orig_nr_pages, &folio->_large_mapcount); break; case RMAP_LEVEL_PMD: @@ -1527,7 +1527,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, enum rmap_level level) { atomic_t *mapped = &folio->_nr_pages_mapped; - int last, nr = 0, nr_pmdmapped = 0; + int last = 0, nr = 0, nr_pmdmapped = 0; bool partially_mapped = false; __folio_rmap_sanity_checks(folio, page, nr_pages, level); @@ -1541,14 +1541,13 @@ static __always_inline void __folio_remove_rmap(struct folio *folio, atomic_sub(nr_pages, &folio->_large_mapcount); do { - last = atomic_add_negative(-1, &page->_mapcount); - if (last) { - last = atomic_dec_return_relaxed(mapped); - if (last < ENTIRELY_MAPPED) - nr++; - } + last += atomic_add_negative(-1, &page->_mapcount); } while (page++, --nr_pages > 0); + if (last && + atomic_sub_return_relaxed(last, mapped) < ENTIRELY_MAPPED) + nr = last; + partially_mapped = nr && atomic_read(mapped); break; case RMAP_LEVEL_PMD: -- 2.45.2
