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

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Date:   Mon, 20 Dec 2021 13:27:38 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.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 Mon, Dec 20, 2021 at 1:02 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> Hah, that was actually how I did it originally (without actually
> committing at each step, and with a few "Oh, hang on, now we can avoid
> calculating this too" stops and restarts along the way), but I thought
> it all hung together logically as a single change.  It's hard to see
> things from the other person's perspective at times.

In just about any other area, I wouldn't mind one bigger patch that
just removes code that isn't used.

But when it's in the vm code, and it's pretty grotty, I do prefer
seeing three patches that individually are much easier to see that
"yeah, this doesn't actually change anything at all".

The combined patch may be exactly the same thing, it's just much
harder to see that "oh, now it's not used any more".

That was perhaps especially true since a number of the changes also
ended up doing statement simplification when the old layout made no
sense any more with part of the results not used.

So your 3-patch series was much easier to look at and go "Yeah, I
believe each of these patches is a no-op".

So ACK on all those patches.

          Linus