linux-kernel - Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG

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Date:   Tue, 21 Dec 2021 10:58:42 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Jan Kara <jack@...e.cz>, Jason Gunthorpe <jgg@...dia.com>,
        Nadav Amit <namit@...are.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Linux-MM <linux-mm@...ck.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)

On Tue, Dec 21, 2021 at 10:51 AM David Hildenbrand <david@...hat.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