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

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Date:   Fri, 17 Dec 2021 23:53:20 +0000
From:   Nadav Amit <namit@...are.com>
To:     David Hildenbrand <david@...hat.com>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        Jason Gunthorpe <jgg@...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>, 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 Dec 17, 2021, at 3:29 PM, David Hildenbrand <david@...hat.com> wrote:
> 
> On 17.12.21 23:58, Linus Torvalds wrote:
>> 
>> And you can't even do the optimistic case without taking the lock,
>> because in your world, a COW that optimistically copies in the case of
>> a race condition is fundamentally *wrong* and buggy. Because in your
>> world-view, GUP and COW are very different and have different rules,
>> but you need things to be *exact*, and they aren’t.

I understand the discussion mainly revolves correctness, which is
obviously the most important property, but I would like to mention
that having transient get_page() calls causing unnecessary COWs can
cause hard-to-analyze and hard-to-avoid performance degradation. COW
means a page copy, a TLB flush and potentially a TLB shootdown, which
is the most painful, specifically on VMs.

So I think that any solution should be able to limit the cases/number
of unnecessary COW operations to be minimal.