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

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Message-ID: <CAHk-=wj+VpgJ3RfRQNxYS3xN9O01rwWnSBX7mztxFaE6BTLzFw@mail.gmail.com>
Date:   Fri, 17 Dec 2021 18:42:11 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Nadav Amit <namit@...are.com>
Cc:     Jason Gunthorpe <jgg@...dia.com>,
        David Hildenbrand <david@...hat.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>,
        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 Fri, Dec 17, 2021 at 6:17 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> I think the hugepage code should use the exact same logic that the
> regular wp fault code does.

IOW, I think that this stupid (AND UNTESTED) patch should likely just
fix David's test-case with the hugepage and splice thing..

Or at least be somewhat close.  But it should be paired with the GUP
side doing the right thing too, of course. Maybe it already does,
maybe it doesn't, I didn't check...

And maybe there's something subtle about the page_count() of a THP
entry. Again, I can't really claim to have tested this all, but I'm
hoping this makes somebody go "Ahh, now I see what Linus means"

            Linus

View attachment "patch.diff" of type "text/x-patch" (1277 bytes)