linux-kernel - Re: ksm: initialize rmap values directly and make them const

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Message-ID: <20260206163850402hEblYPDgLSG4M7CT1ju2G@zte.com.cn>
Date: Fri, 6 Feb 2026 16:38:50 +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: ksm: initialize rmap values directly and make them const

> > Considering that commit 06fbd555dea8 ("ksm: optimize rmap_walk_ksm by passing
> > a suitable addressrange") seems to have already been merged, this new patch is
> > proposed to address the issue raised by David at:
> > 
> > https://lore.kernel.org/all/ba03780a-fd65-4a03-97de-bc0905106260@kernel.org/
> > 
> > This initialize rmap values (addr, pgoff_start, pgoff_end) directly and
> > make them const to make code more robust. Besides, since KSM folios are always
> > order-0, so folio_nr_pages(KSM folio) is always 1, so the line:
> > 
> > 	"pgoff_end = pgoff_start + folio_nr_pages(folio) - 1;"
> > 
> > becomes directly:
> > 
> > 	"pgoff_end = pgoff_start;"
> > 
> > The test reproducer of rmap_walk_ksm can be found at:
> > https://lore.kernel.org/all/20260206151424734QIyWL_pA-1QeJPbJlUxsO@zte.com.cn/
> 
> Thanks!
> 
> > 
> > Fixes: 06fbd555dea8 ("ksm: optimize rmap_walk_ksm by passing a suitable addressrange")
> > Signed-off-by: xu xin <xu.xin16@....com.cn>
> 
> The patch does not seem to be upstream / in mm-stable yet.
> 
> Can you resend the original patch with these changes included and the 
> reproducer referenced in the updated patch description?

Sure, I thought the original patch was merged in linux-next.

> 
> Thanks!
> 
> > ---
> >   mm/ksm.c | 13 +++++--------
> >   1 file changed, 5 insertions(+), 8 deletions(-)
> > 
> > diff --git a/mm/ksm.c b/mm/ksm.c
> > index 031c17e4ada6..c7ca117024a4 100644
> > --- a/mm/ksm.c
> > +++ b/mm/ksm.c
> > @@ -3171,8 +3171,11 @@ void rmap_walk_ksm(struct folio *folio, struct rmap_walk_control *rwc)
> >   		struct anon_vma *anon_vma = rmap_item->anon_vma;
> >   		struct anon_vma_chain *vmac;
> >   		struct vm_area_struct *vma;
> > -		unsigned long addr;
> > -		pgoff_t pgoff_start, pgoff_end;
> > +		/* 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;
> 
> I would move them all the way up, above the "struct anon_vma *anon_vma = 
> rmap_item->anon_vma;"

Yes, that looks better indeed.

> 
> -- 
> Cheers,
> 
> David