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

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Date:   Mon, 20 Dec 2021 11:15:14 -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 10:37 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> This might be a step in the right direction?
>
> Subject: [PATCH] mm: reuse_swap_page() no longer needs to return map_swapcount

Well, that patch seems to be a no-op removal of dead code, so absolutely yes.

That said, I think it would be good to split it up. I looked at that
patch and went "is that really a no-op" to the point of recreating it.

I think it would be good to make it multiple patches that are each
individally trivial. IOW, start with

 (1) remove second argument to reuse_swap_page() that is always NULL,
without making any other changes

 (2) that now made 'total_mapcount' unused in reuse_swap_page(),
remove it as an argument from page_trans_huge_map_swapcount()

 (3) that now made 'total_mapcount' unused in
page_trans_huge_mapcount(), remove it as an argument there too.

because as it stands, that patch of yours looks like it is changing a
lot of things, and I think it would be clearer to remove one thign at
a time as it becomes obviously not used.

Hmm?

           Linus