linux-kernel - Re: [PATCH v2 2/2] ksm: Optimize rmap_walk_ksm by passing a suitable address range

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Message-ID: <20260207001528370-qf6PP2QGXNvcjGdsLTC_@zte.com.cn>
Date: Sat, 7 Feb 2026 00:15:28 +0800 (CST)
From: <xu.xin16@....com.cn>
To: <david@...nel.org>
Cc: <akpm@...ux-foundation.org>, <chengming.zhou@...ux.dev>,
        <hughd@...gle.com>, <wang.yaxin@....com.cn>, <yang.yang29@....com.cn>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] ksm: Optimize rmap_walk_ksm by passing a suitable address range

> s/Reaon/Reason/ or better "Cause"

Thanks. Fix it in v3.

> > In our real embedded Linux environment, the measured metrcis were as follows:
> > 
> > 1) Time_ms: Max time for holding anon_vma lock in a single rmap_walk_ksm.
> > 2) Nr_iteration_total: The max times of iterations in a loop of anon_vma_interval_tree_foreach
> > 3) Skip_addr_out_of_range: The max times of skipping due to the first check (vma->vm_start
> >              and vma->vm_end) in a loop of anon_vma_interval_tree_foreach.
> > 4) Skip_mm_mismatch: The max times of skipping due to the second check (rmap_item->mm == vma->vm_mm)
> >              in a loop of anon_vma_interval_tree_foreach.
> > 
> > The result is as follows:
> > 
> >                   Time_ms      Nr_iteration_total    Skip_addr_out_of_range   Skip_mm_mismatch
> > Before patched:  228.65       22169                 22168                    0
> > After pacthed:   0.396        3                     0                        2
> 
> s/pacthed/patched/
> 
> But I would just call it "Before" and "After".

Thanks.

> 
> > 
> > The referenced reproducer of rmap_walk_ksm can be found at:
> > https://lore.kernel.org/all/20260206151424734QIyWL_pA-1QeJPbJlUxsO@zte.com.cn/
> > 
> > Signed-off-by: xu xin <xu.xin16@....com.cn>
> 
> Did you accidentally drop a
> 
> 	Co-developed-by: Wang Yaxin <wang.yaxin@....com.cn>
> 
> ?

Oh, yes, Thanks.

> 
> > ---
> >   mm/ksm.c | 5 ++++-
> >   1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/ksm.c b/mm/ksm.c
> > index 950e122bcbf4..54f72e92b7f3 100644
> > --- a/mm/ksm.c
> > +++ b/mm/ksm.c
> > @@ -3170,6 +3170,9 @@ void rmap_walk_ksm(struct folio *folio, struct rmap_walk_control *rwc)
> >   	hlist_for_each_entry(rmap_item, &stable_node->hlist, hlist) {
> >   		/* Ignore the stable/unstable/sqnr flags */
> >   		const unsigned long addr = rmap_item->address & PAGE_MASK;
> > +		const pgoff_t pgoff_start = rmap_item->address >> PAGE_SHIFT;
> > +		/* KSM folios are always order-0 normal pages */
> > +		const pgoff_t pgoff_end = pgoff_start;
> 
> 
> Maybe simply
> 
> const pgoff_t pgoff = rmap_item->address >> PAGE_SHIFT;
> 
> and drop pgoff_end? Then you simply pass pgoff as start and end below. 
> You could add the KSM folio comment above the 
> anon_vma_interval_tree_foreach.

Will do it in v3.

> 
> 
> If the tools/testing/selftests/mm/rmap.c selftests keeps passing 
> rmap_walk_ksm() should be working as expected. Did you run it to make sure?

Yes, if running tools/testing/selftests/mm/rmap.c, rmap_walk_ksm() can work, but
it won't trigger a high delay of time consuming on anon_vma_interval_tree_foreach
because there's few VMAs sharing anon_vma.