linux-kernel - Re: [PATCH 1/6] mm: ksm: use more folio api in ksm_might_need_to

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Date:   Wed, 8 Nov 2023 13:59:48 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 1/6] mm: ksm: use more folio api in
 ksm_might_need_to_copy()

On Wed, Nov 08, 2023 at 09:40:09AM +0800, Kefeng Wang wrote:
> 
> 
> On 2023/11/7 22:24, Matthew Wilcox wrote:
> > On Tue, Nov 07, 2023 at 09:52:11PM +0800, Kefeng Wang wrote:
> > >   struct page *ksm_might_need_to_copy(struct page *page,
> > > -			struct vm_area_struct *vma, unsigned long address)
> > > +			struct vm_area_struct *vma, unsigned long addr)
> > >   {
> > >   	struct folio *folio = page_folio(page);
> > >   	struct anon_vma *anon_vma = folio_anon_vma(folio);
> > > -	struct page *new_page;
> > > +	struct folio *new_folio;
> > > -	if (PageKsm(page)) {
> > > -		if (page_stable_node(page) &&
> > > +	if (folio_test_ksm(folio)) {
> > > +		if (folio_stable_node(folio) &&
> > >   		    !(ksm_run & KSM_RUN_UNMERGE))
> > >   			return page;	/* no need to copy it */
> > >   	} else if (!anon_vma) {
> > >   		return page;		/* no need to copy it */
> > > -	} else if (page->index == linear_page_index(vma, address) &&
> > > +	} else if (page->index == linear_page_index(vma, addr) &&
> > 
> > Hmm.  page->index is going away.  What should we do here instead?
> 
> Do you mean to replace page->index to folio->index, or kill index from
> struct page?

I'm asking you what we should do.

Tail pages already don't have a valid ->index (or ->mapping).
So presumably we can't see a tail page here today.  But will we in future?

Just to remind you, the goal here is:

struct page {
	unsigned long memdesc;
};

so folios will be the only thing that have a ->index.  I haven't looked
at this code; I know nothing about it.  But you're changing it, so you
must have some understanding of it.