linux-kernel - Re: [PATCH v2 2/3] userfaultfd: UFFDIO

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Date:   Thu, 28 Sep 2023 12:24:53 -0400
From:   Peter Xu <peterx@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Jann Horn <jannh@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        akpm@...ux-foundation.org, viro@...iv.linux.org.uk,
        brauner@...nel.org, shuah@...nel.org, aarcange@...hat.com,
        lokeshgidra@...gle.com, hughd@...gle.com, mhocko@...e.com,
        axelrasmussen@...gle.com, rppt@...nel.org, willy@...radead.org,
        Liam.Howlett@...cle.com, zhangpeng362@...wei.com,
        bgeffon@...gle.com, kaleshsingh@...gle.com, ngeoffray@...gle.com,
        jdduke@...gle.com, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v2 2/3] userfaultfd: UFFDIO_REMAP uABI

On Wed, Sep 27, 2023 at 03:29:35PM +0200, David Hildenbrand wrote:
> > > +       if (!pte_same(*src_pte, orig_src_pte) ||
> > > +           !pte_same(*dst_pte, orig_dst_pte) ||
> > > +           folio_test_large(src_folio) ||
> > > +           folio_estimated_sharers(src_folio) != 1) {
> 
> ^ here you should check PageAnonExclusive. Please get rid of any implicit
> explicit/implcit mapcount checks.

David, is PageAnon 100% accurate now in the current tree?

IOW, can it be possible that the page has total_mapcount==1 but missing
AnonExclusive bit in any possible way?

Thanks,

-- 
Peter Xu