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

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Message-ID: <CAHk-=wge4Pcrhg-uCyCRVaUBb5B4NdSxb3TNhB5pnnqDqw6Ztg@mail.gmail.com>
Date:   Fri, 17 Dec 2021 12:43:42 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     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>,
        Nadav Amit <namit@...are.com>, 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 Fri, Dec 17, 2021 at 12:39 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> At the time of such a shared pin, you can do what we already do:
> re-use the page if it has a refcount of 1. Or do an early COW event
> (feel free to avoid the "mark it writable and dirty").

Note that this also depends on fork() doing the right thing, marking
things for "a fork() can not share this page any more".

Which it does for regular pages, and is exactly what that
page_needs_cow_for_dma() logic is all about (and the special
write_protect_seq around gup/fork).

I do believe that huge-pages don't do it right. But I think that as
you try to fix hugepages, you are now breaking the normal case.

If all your logic was only about hugepages, I wouldn't care so much.
But you are playing questionable games with code that I think is
correct.

Please explain why.

              Linus