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Date: Mon, 17 Jan 2022 07:14:53 -0500 From: Johannes Weiner <hannes@...xchg.org> To: cgel.zte@...il.com Cc: akpm@...ux-foundation.org, sfr@...b.auug.org.au, linux-mm@...ck.org, linux-kernel@...r.kernel.org, Yang Yang <yang.yang29@....com.cn> Subject: Re: [PATCH] psi: Treat ksm swapping in copy as memstall Hello Yang, On Sun, Jan 16, 2022 at 03:21:51PM +0000, cgel.zte@...il.com wrote: > From: Yang Yang <yang.yang29@....com.cn> > > When faults in from swap what used to be a ksm page and that page > had been swapped in before, system has to make a copy. Obviously > this kind of copy is related to high memory pressure, so we treat > it as memstall. Although ksm page merging is not because of high > memory pressure. > > Information of this new kind of stall will help psi to account > memory pressure more precise. Thanks for your patch. I'm curious if you have a concrete use case where this makes a difference, or if this is something you found while reading the code? > Signed-off-by: Yang Yang <yang.yang29@....com.cn> > --- > mm/ksm.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/mm/ksm.c b/mm/ksm.c > index 4a7f8614e57d..d4ec6773f9b8 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -39,6 +39,7 @@ > #include <linux/freezer.h> > #include <linux/oom.h> > #include <linux/numa.h> > +#include <linux/psi.h> > > #include <asm/tlbflush.h> > #include "internal.h" > @@ -2569,6 +2570,7 @@ struct page *ksm_might_need_to_copy(struct page *page, > { > struct anon_vma *anon_vma = page_anon_vma(page); > struct page *new_page; > + unsigned long pflags; > > if (PageKsm(page)) { > if (page_stable_node(page) && > @@ -2583,6 +2585,7 @@ struct page *ksm_might_need_to_copy(struct page *page, > if (!PageUptodate(page)) > return page; /* let do_swap_page report the error */ > > + psi_memstall_enter(&pflags); > new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address); > if (new_page && > mem_cgroup_charge(page_folio(new_page), vma->vm_mm, GFP_KERNEL)) { > @@ -2600,6 +2603,7 @@ struct page *ksm_might_need_to_copy(struct page *page, > #endif > } > > + psi_memstall_leave(&pflags); This does unconditional stall accounting for a swapin operation. But if you take a look at workingset_refault() -> folio_wait_bit_common(), we only count memory stalls when the page is thrashing, not when it's a transitionary refault (which happen even when there is enough memory to hold the workingset). You need to check PageWorkingset() at least. But again I'd be curious first if this is a practical concern. Swapins should be IO dominated - or in the case of zswap dominated by the decompression. Does a page copy really matter?
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