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Date: Thu, 20 May 2021 09:59:15 +0800 From: "Huang, Ying" <ying.huang@...el.com> To: Johannes Weiner <hannes@...xchg.org> Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Peter Xu <peterx@...hat.com>, Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...riel.com>, Andrea Arcangeli <aarcange@...hat.com>, Michal Hocko <mhocko@...nel.org>, Dave Hansen <dave.hansen@...el.com>, Tim Chen <tim.c.chen@...el.com> Subject: Re: [PATCH] mm: move idle swap cache pages to the tail of LRU after COW Johannes Weiner <hannes@...xchg.org> writes: > On Thu, May 20, 2021 at 09:22:45AM +0800, Huang, Ying wrote: >> Johannes Weiner <hannes@...xchg.org> writes: >> >> > On Wed, May 19, 2021 at 09:33:13AM +0800, Huang Ying wrote: >> >> diff --git a/mm/memory.c b/mm/memory.c >> >> index b83f734c4e1d..2b6847f4c03e 100644 >> >> --- a/mm/memory.c >> >> +++ b/mm/memory.c >> >> @@ -3012,6 +3012,11 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf) >> >> munlock_vma_page(old_page); >> >> unlock_page(old_page); >> >> } >> >> + if (page_copied && PageSwapCache(old_page) && >> >> + !page_mapped(old_page) && trylock_page(old_page)) { >> >> + try_to_free_idle_swapcache(old_page); >> >> + unlock_page(old_page); >> > >> > If there are no more swap or pte references, can we just attempt to >> > free the page right away, like we do during regular unmap? >> > >> > if (page_copied) >> > free_swap_cache(old_page); >> > put_page(old_page); >> >> A previous version of the patch does roughly this. >> >> https://lore.kernel.org/lkml/20210113024241.179113-1-ying.huang@intel.com/ >> >> But Linus has concerns with the overhead introduced in the hot COW path. > > Sorry, I had missed that thread. > > It sounds like there were the same concerns about the LRU shuffling > overhead in the COW page. Now we have numbers for that, but not the > free_swap_cache version. Would you be able to run the numbers for that > as well? It would be interesting to see how much the additional code > complexity buys us. The number for which workload? The workload that is used to evaluate this patch? >> Another possibility is to move the idle swap cache page to the tail of >> the file LRU list. But the question is how to identify the page. > > The LRU type is identified by PG_swapbacked, and we do clear that for > anon pages to implement MADV_FREE. It may work here too. But I'm > honestly a bit skeptical about the ROI on this... The definition of PageSwapCache() is static __always_inline int PageSwapCache(struct page *page) { #ifdef CONFIG_THP_SWAP page = compound_head(page); #endif return PageSwapBacked(page) && test_bit(PG_swapcache, &page->flags); } So we cannot clear PG_swapbacked directly. Best Regards, Huang, Ying
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