Approximately 10-20% of pages to be swapped out are zero pages [1]. Rather than reading/writing these pages to flash resulting in increased I/O and flash wear, the pte can be cleared for those addresses at unmap time while shrinking folio list. When this causes a page fault, do_pte_missing will take care of this page. With this patch, NVMe writes in Meta server fleet decreased by almost 10% with conventional swap setup (zswap disabled). [1] https://lore.kernel.org/all/20171018104832epcms5p1b2232e2236258de3d03d1344dde9fce0@epcms5p1/ Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx> --- include/linux/rmap.h | 1 + mm/rmap.c | 163 ++++++++++++++++++++++--------------------- mm/vmscan.c | 89 ++++++++++++++++------- 3 files changed, 150 insertions(+), 103 deletions(-) diff --git a/include/linux/rmap.h b/include/linux/rmap.h index bb53e5920b88..b36db1e886e4 100644 --- a/include/linux/rmap.h +++ b/include/linux/rmap.h @@ -100,6 +100,7 @@ enum ttu_flags { * do a final flush if necessary */ TTU_RMAP_LOCKED = 0x80, /* do not grab rmap lock: * caller holds it */ + TTU_ZERO_FOLIO = 0x100,/* zero folio */ }; #ifdef CONFIG_MMU diff --git a/mm/rmap.c b/mm/rmap.c index 52357d79917c..d98f70876327 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1819,96 +1819,101 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, */ dec_mm_counter(mm, mm_counter(folio)); } else if (folio_test_anon(folio)) { - swp_entry_t entry = page_swap_entry(subpage); - pte_t swp_pte; - /* - * Store the swap location in the pte. - * See handle_pte_fault() ... - */ - if (unlikely(folio_test_swapbacked(folio) != - folio_test_swapcache(folio))) { + if (flags & TTU_ZERO_FOLIO) { + pte_clear(mm, address, pvmw.pte); + dec_mm_counter(mm, MM_ANONPAGES); + } else { + swp_entry_t entry = page_swap_entry(subpage); + pte_t swp_pte; /* - * unmap_huge_pmd_locked() will unmark a - * PMD-mapped folio as lazyfree if the folio or - * its PMD was redirtied. + * Store the swap location in the pte. + * See handle_pte_fault() ... */ - if (!pmd_mapped) - WARN_ON_ONCE(1); - goto walk_done_err; - } + if (unlikely(folio_test_swapbacked(folio) != + folio_test_swapcache(folio))) { + /* + * unmap_huge_pmd_locked() will unmark a + * PMD-mapped folio as lazyfree if the folio or + * its PMD was redirtied. + */ + if (!pmd_mapped) + WARN_ON_ONCE(1); + goto walk_done_err; + } - /* MADV_FREE page check */ - if (!folio_test_swapbacked(folio)) { - int ref_count, map_count; + /* MADV_FREE page check */ + if (!folio_test_swapbacked(folio)) { + int ref_count, map_count; - /* - * Synchronize with gup_pte_range(): - * - clear PTE; barrier; read refcount - * - inc refcount; barrier; read PTE - */ - smp_mb(); + /* + * Synchronize with gup_pte_range(): + * - clear PTE; barrier; read refcount + * - inc refcount; barrier; read PTE + */ + smp_mb(); - ref_count = folio_ref_count(folio); - map_count = folio_mapcount(folio); + ref_count = folio_ref_count(folio); + map_count = folio_mapcount(folio); - /* - * Order reads for page refcount and dirty flag - * (see comments in __remove_mapping()). - */ - smp_rmb(); + /* + * Order reads for page refcount and dirty flag + * (see comments in __remove_mapping()). + */ + smp_rmb(); - /* - * The only page refs must be one from isolation - * plus the rmap(s) (dropped by discard:). - */ - if (ref_count == 1 + map_count && - !folio_test_dirty(folio)) { - dec_mm_counter(mm, MM_ANONPAGES); - goto discard; - } + /* + * The only page refs must be one from isolation + * plus the rmap(s) (dropped by discard:). + */ + if (ref_count == 1 + map_count && + !folio_test_dirty(folio)) { + dec_mm_counter(mm, MM_ANONPAGES); + goto discard; + } - /* - * If the folio was redirtied, it cannot be - * discarded. Remap the page to page table. - */ - set_pte_at(mm, address, pvmw.pte, pteval); - folio_set_swapbacked(folio); - goto walk_done_err; - } + /* + * If the folio was redirtied, it cannot be + * discarded. Remap the page to page table. + */ + set_pte_at(mm, address, pvmw.pte, pteval); + folio_set_swapbacked(folio); + goto walk_done_err; + } - if (swap_duplicate(entry) < 0) { - set_pte_at(mm, address, pvmw.pte, pteval); - goto walk_done_err; - } - if (arch_unmap_one(mm, vma, address, pteval) < 0) { - swap_free(entry); - set_pte_at(mm, address, pvmw.pte, pteval); - goto walk_done_err; - } + if (swap_duplicate(entry) < 0) { + set_pte_at(mm, address, pvmw.pte, pteval); + goto walk_done_err; + } + if (arch_unmap_one(mm, vma, address, pteval) < 0) { + swap_free(entry); + set_pte_at(mm, address, pvmw.pte, pteval); + goto walk_done_err; + } - /* See folio_try_share_anon_rmap(): clear PTE first. */ - if (anon_exclusive && - folio_try_share_anon_rmap_pte(folio, subpage)) { - swap_free(entry); - set_pte_at(mm, address, pvmw.pte, pteval); - goto walk_done_err; - } - if (list_empty(&mm->mmlist)) { - spin_lock(&mmlist_lock); - if (list_empty(&mm->mmlist)) - list_add(&mm->mmlist, &init_mm.mmlist); - spin_unlock(&mmlist_lock); + /* See folio_try_share_anon_rmap(): clear PTE first. */ + if (anon_exclusive && + folio_try_share_anon_rmap_pte(folio, subpage)) { + swap_free(entry); + set_pte_at(mm, address, pvmw.pte, pteval); + goto walk_done_err; + } + if (list_empty(&mm->mmlist)) { + spin_lock(&mmlist_lock); + if (list_empty(&mm->mmlist)) + list_add(&mm->mmlist, &init_mm.mmlist); + spin_unlock(&mmlist_lock); + } + dec_mm_counter(mm, MM_ANONPAGES); + inc_mm_counter(mm, MM_SWAPENTS); + swp_pte = swp_entry_to_pte(entry); + if (anon_exclusive) + swp_pte = pte_swp_mkexclusive(swp_pte); + if (pte_soft_dirty(pteval)) + swp_pte = pte_swp_mksoft_dirty(swp_pte); + if (pte_uffd_wp(pteval)) + swp_pte = pte_swp_mkuffd_wp(swp_pte); + set_pte_at(mm, address, pvmw.pte, swp_pte); } - dec_mm_counter(mm, MM_ANONPAGES); - inc_mm_counter(mm, MM_SWAPENTS); - swp_pte = swp_entry_to_pte(entry); - if (anon_exclusive) - swp_pte = pte_swp_mkexclusive(swp_pte); - if (pte_soft_dirty(pteval)) - swp_pte = pte_swp_mksoft_dirty(swp_pte); - if (pte_uffd_wp(pteval)) - swp_pte = pte_swp_mkuffd_wp(swp_pte); - set_pte_at(mm, address, pvmw.pte, swp_pte); } else { /* * This is a locked file-backed folio, diff --git a/mm/vmscan.c b/mm/vmscan.c index b9170f767353..d54f44b556f0 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -1026,6 +1026,38 @@ static bool may_enter_fs(struct folio *folio, gfp_t gfp_mask) return !data_race(folio_swap_flags(folio) & SWP_FS_OPS); } +static bool is_folio_page_zero_filled(struct folio *folio, int i) +{ + unsigned long *data; + unsigned int pos, last_pos = PAGE_SIZE / sizeof(*data) - 1; + bool ret = false; + + data = kmap_local_folio(folio, i * PAGE_SIZE); + + if (data[last_pos]) + goto out; + + for (pos = 0; pos < last_pos; pos++) { + if (data[pos]) + goto out; + } + ret = true; +out: + kunmap_local(data); + return ret; +} + +static bool is_folio_zero_filled(struct folio *folio) +{ + unsigned int i; + + for (i = 0; i < folio_nr_pages(folio); i++) { + if (!is_folio_page_zero_filled(folio, i)) + return false; + } + return true; +} + /* * shrink_folio_list() returns the number of reclaimed pages */ @@ -1053,6 +1085,7 @@ static unsigned int shrink_folio_list(struct list_head *folio_list, enum folio_references references = FOLIOREF_RECLAIM; bool dirty, writeback; unsigned int nr_pages; + bool folio_zero_filled = false; cond_resched(); @@ -1270,6 +1303,7 @@ static unsigned int shrink_folio_list(struct list_head *folio_list, nr_pages = 1; } + folio_zero_filled = is_folio_zero_filled(folio); /* * The folio is mapped into the page tables of one or more * processes. Try to unmap it here. @@ -1295,6 +1329,9 @@ static unsigned int shrink_folio_list(struct list_head *folio_list, if (folio_test_large(folio) && list_empty(&folio->_deferred_list)) flags |= TTU_SYNC; + if (folio_zero_filled) + flags |= TTU_ZERO_FOLIO; + try_to_unmap(folio, flags); if (folio_mapped(folio)) { stat->nr_unmap_fail += nr_pages; @@ -1358,32 +1395,36 @@ static unsigned int shrink_folio_list(struct list_head *folio_list, * starts and then write it out here. */ try_to_unmap_flush_dirty(); - switch (pageout(folio, mapping, &plug)) { - case PAGE_KEEP: - goto keep_locked; - case PAGE_ACTIVATE: - goto activate_locked; - case PAGE_SUCCESS: - stat->nr_pageout += nr_pages; + if (folio_zero_filled) { + folio_clear_dirty(folio); + } else { + switch (pageout(folio, mapping, &plug)) { + case PAGE_KEEP: + goto keep_locked; + case PAGE_ACTIVATE: + goto activate_locked; + case PAGE_SUCCESS: + stat->nr_pageout += nr_pages; - if (folio_test_writeback(folio)) - goto keep; - if (folio_test_dirty(folio)) - goto keep; + if (folio_test_writeback(folio)) + goto keep; + if (folio_test_dirty(folio)) + goto keep; - /* - * A synchronous write - probably a ramdisk. Go - * ahead and try to reclaim the folio. - */ - if (!folio_trylock(folio)) - goto keep; - if (folio_test_dirty(folio) || - folio_test_writeback(folio)) - goto keep_locked; - mapping = folio_mapping(folio); - fallthrough; - case PAGE_CLEAN: - ; /* try to free the folio below */ + /* + * A synchronous write - probably a ramdisk. Go + * ahead and try to reclaim the folio. + */ + if (!folio_trylock(folio)) + goto keep; + if (folio_test_dirty(folio) || + folio_test_writeback(folio)) + goto keep_locked; + mapping = folio_mapping(folio); + fallthrough; + case PAGE_CLEAN: + ; /* try to free the folio below */ + } } } -- 2.43.0
