From 4e0cf05f60590c4119c2eeacf136d1b832978370 Mon Sep 17 00:00:00 2001 From: Johannes Weiner Date: Tue, 6 Dec 2022 18:13:39 +0100 Subject: mm: memcontrol: skip moving non-present pages that are mapped elsewhere Patch series "mm: push down lock_page_memcg()", v2. This patch (of 3): During charge moving, the pte lock and the page lock cover nearly all cases of stabilizing page_mapped(). The only exception is when we're looking at a non-present pte and find a page in the page cache or in the swapcache: if the page is mapped elsewhere, it can become unmapped outside of our control. For this reason, rmap needs lock_page_memcg(). We don't like cgroup-specific locks in generic MM code - especially in performance-critical MM code - and for a legacy feature that's unlikely to have many users left - if any. So remove the exception. Arguably that's better semantics anyway: the page is shared, and another process seems to be the more active user. Once we stop moving such pages, rmap doesn't need lock_page_memcg() anymore. The next patch will remove it. Link: https://lkml.kernel.org/r/20221206171340.139790-1-hannes@cmpxchg.org Link: https://lkml.kernel.org/r/20221206171340.139790-2-hannes@cmpxchg.org Signed-off-by: Johannes Weiner Suggested-by: Hugh Dickins Acked-by: Hugh Dickins Acked-by: Shakeel Butt Acked-by: Michal Hocko Cc: Roman Gushchin Cc: Muchun Song Signed-off-by: Andrew Morton --- mm/memcontrol.c | 52 ++++++++++++++++++++++++++++++++++++++-------------- 1 file changed, 38 insertions(+), 14 deletions(-) (limited to 'mm/memcontrol.c') diff --git a/mm/memcontrol.c b/mm/memcontrol.c index ab457f0394ab..a698a2b6523b 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -5692,7 +5692,7 @@ static struct page *mc_handle_file_pte(struct vm_area_struct *vma, * @from: mem_cgroup which the page is moved from. * @to: mem_cgroup which the page is moved to. @from != @to. * - * The caller must make sure the page is not on LRU (isolate_page() is useful.) + * The page must be locked and not on the LRU. * * This function doesn't do "charge" to new cgroup and doesn't do "uncharge" * from old cgroup. @@ -5709,20 +5709,13 @@ static int mem_cgroup_move_account(struct page *page, int nid, ret; VM_BUG_ON(from == to); + VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio); VM_BUG_ON_FOLIO(folio_test_lru(folio), folio); VM_BUG_ON(compound && !folio_test_large(folio)); - /* - * Prevent mem_cgroup_migrate() from looking at - * page's memory cgroup of its source page while we change it. - */ - ret = -EBUSY; - if (!folio_trylock(folio)) - goto out; - ret = -EINVAL; if (folio_memcg(folio) != from) - goto out_unlock; + goto out; pgdat = folio_pgdat(folio); from_vec = mem_cgroup_lruvec(from, pgdat); @@ -5809,8 +5802,6 @@ static int mem_cgroup_move_account(struct page *page, mem_cgroup_charge_statistics(from, -nr_pages); memcg_check_events(from, nid); local_irq_enable(); -out_unlock: - folio_unlock(folio); out: return ret; } @@ -5859,6 +5850,29 @@ static enum mc_target_type get_mctgt_type(struct vm_area_struct *vma, else if (is_swap_pte(ptent)) page = mc_handle_swap_pte(vma, ptent, &ent); + if (target && page) { + if (!trylock_page(page)) { + put_page(page); + return ret; + } + /* + * page_mapped() must be stable during the move. This + * pte is locked, so if it's present, the page cannot + * become unmapped. If it isn't, we have only partial + * control over the mapped state: the page lock will + * prevent new faults against pagecache and swapcache, + * so an unmapped page cannot become mapped. However, + * if the page is already mapped elsewhere, it can + * unmap, and there is nothing we can do about it. + * Alas, skip moving the page in this case. + */ + if (!pte_present(ptent) && page_mapped(page)) { + unlock_page(page); + put_page(page); + return ret; + } + } + if (!page && !ent.val) return ret; if (page) { @@ -5875,8 +5889,11 @@ static enum mc_target_type get_mctgt_type(struct vm_area_struct *vma, if (target) target->page = page; } - if (!ret || !target) + if (!ret || !target) { + if (target) + unlock_page(page); put_page(page); + } } /* * There is a swap entry and a page doesn't exist or isn't charged. @@ -5916,6 +5933,10 @@ static enum mc_target_type get_mctgt_type_thp(struct vm_area_struct *vma, ret = MC_TARGET_PAGE; if (target) { get_page(page); + if (!trylock_page(page)) { + put_page(page); + return MC_TARGET_NONE; + } target->page = page; } } @@ -6154,6 +6175,7 @@ static int mem_cgroup_move_charge_pte_range(pmd_t *pmd, } putback_lru_page(page); } + unlock_page(page); put_page(page); } else if (target_type == MC_TARGET_DEVICE) { page = target.page; @@ -6162,6 +6184,7 @@ static int mem_cgroup_move_charge_pte_range(pmd_t *pmd, mc.precharge -= HPAGE_PMD_NR; mc.moved_charge += HPAGE_PMD_NR; } + unlock_page(page); put_page(page); } spin_unlock(ptl); @@ -6204,7 +6227,8 @@ retry: } if (!device) putback_lru_page(page); -put: /* get_mctgt_type() gets the page */ +put: /* get_mctgt_type() gets & locks the page */ + unlock_page(page); put_page(page); break; case MC_TARGET_SWAP: -- cgit v1.2.3