From d4c470970d45c863fafc757521a82be2f80b1232 Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Thu, 24 Mar 2022 18:13:34 -0700 Subject: [PATCH] mm: optimize do_wp_page() for fresh pages in local LRU pagevecs For example, if a page just got swapped in via a read fault, the LRU pagevecs might still hold a reference to the page. If we trigger a write fault on such a page, the additional reference from the LRU pagevecs will prohibit reusing the page. Let's conditionally drain the local LRU pagevecs when we stumble over a !PageLRU() page. We cannot easily drain remote LRU pagevecs and it might not be desirable performance-wise. Consequently, this will only avoid copying in some cases. Add a simple "page_count(page) > 3" check first but keep the "page_count(page) > 1 + PageSwapCache(page)" check in place, as we want to minimize cases where we remove a page from the swapcache but won't be able to reuse it, for example, because another process has it mapped R/O, to not affect reclaim. We cannot easily handle the following cases and we will always have to copy: (1) The page is referenced in the LRU pagevecs of other CPUs. We really would have to drain the LRU pagevecs of all CPUs -- most probably copying is much cheaper. (2) The page is already PageLRU() but is getting moved between LRU lists, for example, for activation (e.g., mark_page_accessed()), deactivation (MADV_COLD), or lazyfree (MADV_FREE). We'd have to drain mostly unconditionally, which might be bad performance-wise. Most probably this won't happen too often in practice. Note that there are other reasons why an anon page might temporarily not be PageLRU(): for example, compaction and migration have to isolate LRU pages from the LRU lists first (isolate_lru_page()), moving them to temporary local lists and clearing PageLRU() and holding an additional reference on the page. In that case, we'll always copy. This change seems to be fairly effective with the reproducer [1] shared by Nadav, as long as writeback is done synchronously, for example, using zram. However, with asynchronous writeback, we'll usually fail to free the swapcache because the page is still under writeback: something we cannot easily optimize for, and maybe it's not really relevant in practice. [1] https://lkml.kernel.org/r/0480D692-D9B2-429A-9A88-9BBA1331AC3A@gmail.com Link: https://lkml.kernel.org/r/20220131162940.210846-3-david@redhat.com Signed-off-by: David Hildenbrand Acked-by: Vlastimil Babka Cc: Andrea Arcangeli Cc: Christoph Hellwig Cc: David Rientjes Cc: Don Dutile Cc: Hugh Dickins Cc: Jan Kara Cc: Jann Horn Cc: Jason Gunthorpe Cc: John Hubbard Cc: Kirill A. Shutemov Cc: Liang Zhang Cc: Matthew Wilcox (Oracle) Cc: Michal Hocko Cc: Mike Kravetz Cc: Mike Rapoport Cc: Nadav Amit Cc: Oleg Nesterov Cc: Peter Xu Cc: Rik van Riel Cc: Roman Gushchin Cc: Shakeel Butt Cc: Yang Shi Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/memory.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/mm/memory.c b/mm/memory.c index 5129f3e..c7723bd 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3294,7 +3294,15 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf) * * PageKsm() doesn't necessarily raise the page refcount. */ - if (PageKsm(page) || page_count(page) > 1 + PageSwapCache(page)) + if (PageKsm(page) || page_count(page) > 3) + goto copy; + if (!PageLRU(page)) + /* + * Note: We cannot easily detect+handle references from + * remote LRU pagevecs or references to PageLRU() pages. + */ + lru_add_drain(); + if (page_count(page) > 1 + PageSwapCache(page)) goto copy; if (!trylock_page(page)) goto copy; -- 2.7.4