linux-kernel - Re: [PATCH v2 06/10] mm: memory: use a folio in zap_pte

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Date:   Mon, 6 Nov 2023 14:20:56 +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>,
        linux-s390@...r.kernel.org
Subject: Re: [PATCH v2 06/10] mm: memory: use a folio in zap_pte_range()

On Mon, Nov 06, 2023 at 10:30:59AM +0800, Kefeng Wang wrote:
> On 2023/11/5 1:20, Matthew Wilcox wrote:
> > > -				page_remove_rmap(page, vma, false);
> > > -			put_page(page);
> > > +				page_remove_rmap(&folio->page, vma, false);
> > > +			folio_put(folio);
> > 
> > This is wrong.  If we have a PTE-mapped THP, you'll remove the head page
> > N times instead of removing each of N pages.
> 
> This is device private entry, I suppose that it won't be a THP and large
> folio when check migrate_vma_check_page() and migrate_vma_insert_page(),
> right?

I don't want to leave that kind of booby-trap in the code.  Both places
which currently call page_remove_rmap() should be left as referring to
the page, not the folio.