Date:   Sun, 19 Dec 2021 22:02:01 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     David Hildenbrand <david@redhat.com>,
        Nadav Amit <namit@vmware.com>,
        Jason Gunthorpe <jgg@nvidia.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>,
        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>, Jan Kara <jack@suse.cz>,
        Linux-MM <linux-mm@kvack.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)
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On Sun, Dec 19, 2021 at 01:53:36PM -0800, Linus Torvalds wrote:
> On Sun, Dec 19, 2021 at 1:48 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > Yes, agreed, I was thinking that we could use "not mapped at all"
> > as an optimisation to avoid doing rmap walks.  eg __unmap_and_move().
> 
> So the thing is, it's a very dodgy optimization for a rather simple
> reason: what if somebody pages the page in?
> 
> So even "not mapped at all" is questionable.
> 
> You have to check that it's also not a swapcache page, and hold the
> page lock for that check, at the very least.
> 
> And by then, you're really in a very unusual situation - and my gut
> feel says not one worth optimizing for (because anon pages are
> _usually_ mapped at least once).

I'd like to get rid of ->mapcount for file pages too.  And those are
definitely never mapped in the majority of cases.