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

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Message-ID: <f3479378-48bc-23be-d651-3ec52bb0085d@redhat.com>
Date:   Sat, 18 Dec 2021 11:06:57 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Nadav Amit <namit@...are.com>
Cc:     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>,
        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 18.12.21 03:42, Linus Torvalds wrote:
> 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"

Not the reaction you are hoping for: "Gah, Linus still doesn't see why
the page_count is just wrong". :)

See the mail I just wrote, let's get a common understanding of how our
check should actually look like.

-- 
Thanks,

David / dhildenb