Message-ID: <f3479378-48bc-23be-d651-3ec52bb0085d@redhat.com>
Date:   Sat, 18 Dec 2021 11:06:57 +0100
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Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)
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To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Nadav Amit <namit@vmware.com>
Cc:     Jason Gunthorpe <jgg@nvidia.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        David Rientjes <rientjes@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Yang Shi <shy828301@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Rik van Riel <riel@surriel.com>,
        Roman Gushchin <guro@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Peter Xu <peterx@redhat.com>,
        Donald Dutile <ddutile@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Oleg Nesterov <oleg@redhat.com>, Jan Kara <jack@suse.cz>,
        Linux-MM <linux-mm@kvack.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
References: <20211217113049.23850-1-david@redhat.com>
 <20211217113049.23850-7-david@redhat.com>
 <CAHk-=wgL5u3XMgfUN6BOqVO0OvPx3-LEri1ju-1TW4dFhHQO4g@mail.gmail.com>
 <CAHk-=wgKft6E_EeLA1GnEXcQBA9vu8m2B-M-U7PuiNa0+9gpHA@mail.gmail.com>
 <54c492d7-ddcd-dcd0-7209-efb2847adf7c@redhat.com>
 <CAHk-=wgjOsHAXttQa=csLG10Cp2hh8Dk8CnNC3_WDpBpTzBESQ@mail.gmail.com>
 <20211217204705.GF6385@nvidia.com>
 <2E28C79D-F79C-45BE-A16C-43678AD165E9@vmware.com>
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 <CAHk-=wj+VpgJ3RfRQNxYS3xN9O01rwWnSBX7mztxFaE6BTLzFw@mail.gmail.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <CAHk-=wj+VpgJ3RfRQNxYS3xN9O01rwWnSBX7mztxFaE6BTLzFw@mail.gmail.com>
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On 18.12.21 03:42, Linus Torvalds wrote:
> On Fri, Dec 17, 2021 at 6:17 PM Linus Torvalds
> <torvalds@linux-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