linux-kernel - Re: [PATCH 3/3] ksm: replace function unmerge_ksm_pages with break

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Message-ID: <udw3ecuflmfmjvfwubtss6ky2kx2g3d4hlhfyl6xlxjkk76asz@x4byag532ooc>
Date: Thu, 30 Oct 2025 09:44:26 -0300
From: Pedro Demarchi Gomes <pedrodemargomes@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Xu Xin <xu.xin16@....com.cn>, Chengming Zhou <chengming.zhou@...ux.dev>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] ksm: replace function unmerge_ksm_pages with
 break_ksm

On Wed, Oct 29, 2025 at 03:46:36PM +0100, David Hildenbrand wrote:
> On 28.10.25 14:19, Pedro Demarchi Gomes wrote:
> > Function unmerge_ksm_pages() is unnecessary since now break_ksm() walks
> > an address range. So replace it with break_ksm().
> > 
> > Suggested-by: David Hildenbrand <david@...hat.com>
> > Signed-off-by: Pedro Demarchi Gomes <pedrodemargomes@...il.com>
> > ---
> >   mm/ksm.c | 39 ++++++++++++++++-----------------------
> >   1 file changed, 16 insertions(+), 23 deletions(-)
> > 
> > diff --git a/mm/ksm.c b/mm/ksm.c
> > index 1d1ef0554c7c..18c9e3bda285 100644
> > --- a/mm/ksm.c
> > +++ b/mm/ksm.c
> > @@ -669,6 +669,18 @@ static const struct mm_walk_ops break_ksm_lock_vma_ops = {
> >   };
> >   /*
> > + * Though it's very tempting to unmerge rmap_items from stable tree rather
> > + * than check every pte of a given vma, the locking doesn't quite work for
> > + * that - an rmap_item is assigned to the stable tree after inserting ksm
> > + * page and upping mmap_lock.  Nor does it fit with the way we skip dup'ing
> > + * rmap_items from parent to child at fork time (so as not to waste time
> > + * if exit comes before the next scan reaches it).
> > + *
> > + * Similarly, although we'd like to remove rmap_items (so updating counts
> > + * and freeing memory) when unmerging an area, it's easier to leave that
> > + * to the next pass of ksmd - consider, for example, how ksmd might be
> > + * in cmp_and_merge_page on one of the rmap_items we would be removing.
> > + *
> >    * We use break_ksm to break COW on a ksm page by triggering unsharing,
> >    * such that the ksm page will get replaced by an exclusive anonymous page.
> >    *
> > @@ -1077,25 +1089,6 @@ static void remove_trailing_rmap_items(struct ksm_rmap_item **rmap_list)
> >   	}
> >   }
> > -/*
> > - * Though it's very tempting to unmerge rmap_items from stable tree rather
> > - * than check every pte of a given vma, the locking doesn't quite work for
> > - * that - an rmap_item is assigned to the stable tree after inserting ksm
> > - * page and upping mmap_lock.  Nor does it fit with the way we skip dup'ing
> > - * rmap_items from parent to child at fork time (so as not to waste time
> > - * if exit comes before the next scan reaches it).
> > - *
> > - * Similarly, although we'd like to remove rmap_items (so updating counts
> > - * and freeing memory) when unmerging an area, it's easier to leave that
> > - * to the next pass of ksmd - consider, for example, how ksmd might be
> > - * in cmp_and_merge_page on one of the rmap_items we would be removing.
> > - */
> > -static int unmerge_ksm_pages(struct vm_area_struct *vma,
> > -			     unsigned long start, unsigned long end, bool lock_vma)
> > -{
> > -	return break_ksm(vma, start, end, lock_vma);
> > -}
> > -
> >   static inline
> >   struct ksm_stable_node *folio_stable_node(const struct folio *folio)
> >   {
> > @@ -1233,7 +1226,7 @@ static int unmerge_and_remove_all_rmap_items(void)
> >   		for_each_vma(vmi, vma) {
> >   			if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
> >   				continue;
> > -			err = unmerge_ksm_pages(vma,
> > +			err = break_ksm(vma,
> >   						vma->vm_start, vma->vm_end, false);
> 
> Move that all into a single line.
> 

Ok.

> 
> With that
> 
> Acked-by: David Hildenbrand <david@...hat.com>
> 
> Thanks for tackling this!
> 

Thanks for your comments!
I will send a v2 soon with the corrections.

> -- 
> Cheers
> 
> David / dhildenb
> 
>