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

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Message-ID: <Yb+r2W6RCKhO5toC@casper.infradead.org>
Date:   Sun, 19 Dec 2021 22:02:01 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Hildenbrand <david@...hat.com>,
        Nadav Amit <namit@...are.com>,
        Jason Gunthorpe <jgg@...dia.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>,
        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>, Jan Kara <jack@...e.cz>,
        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 Sun, Dec 19, 2021 at 01:53:36PM -0800, Linus Torvalds wrote:
> On Sun, Dec 19, 2021 at 1:48 PM Matthew Wilcox <willy@...radead.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.