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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 21 Dec 2021 10:58:42 -0800
Message-ID: <CAHk-=whJeoncgYjzfLOR_dQbJUyOiE1NxE753m1mvAtXZ00tMw@mail.gmail.com>
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)
To:     David Hildenbrand <david@redhat.com>
Cc:     Jan Kara <jack@suse.cz>, Jason Gunthorpe <jgg@nvidia.com>,
        Nadav Amit <namit@vmware.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        David Rientjes <rientjes@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Yang Shi <shy828301@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Rik van Riel <riel@surriel.com>,
        Roman Gushchin <guro@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Peter Xu <peterx@redhat.com>,
        Donald Dutile <ddutile@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Oleg Nesterov <oleg@redhat.com>, Linux-MM <linux-mm@kvack.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
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On Tue, Dec 21, 2021 at 10:51 AM David Hildenbrand <david@redhat.com> wrote:
>
> For that purpose the pincount would already kind-off work. Not precise,
> but at least something ("this page cannot possibly have been pinned").

That part actually exists already, ie put_page_refs() has this:

  #ifdef CONFIG_DEBUG_VM
        if (VM_WARN_ON_ONCE_PAGE(page_ref_count(page) < refs, page))
                return;
  #endif

And yeah, it shouldn't have that '#ifdef CONFIG_DEBUG_VM' there, but I
think it's because the non-CONFIG_DEBUG_VM #define for
VM_WARN_ON_ONCE_PAGE() is broken, and doesn't return 0.

                   Linus