linux-kernel - Re: [PATCH v8 4/6] userfaultfd: add UFFDIO

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Date:   Thu, 25 Feb 2021 15:51:04 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Chinwen Chang <chinwen.chang@...iatek.com>,
        Huang Ying <ying.huang@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Koutný <mkoutny@...e.com>,
        Michel Lespinasse <walken@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Nicholas Piggin <npiggin@...il.com>, Shaohua Li <shli@...com>,
        Shawn Anastasio <shawn@...stas.io>,
        Steven Rostedt <rostedt@...dmis.org>,
        Steven Price <steven.price@....com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        Adam Ruprecht <ruprecht@...gle.com>,
        Cannon Matthews <cannonmatthews@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Oliver Upton <oupton@...gle.com>
Subject: Re: [PATCH v8 4/6] userfaultfd: add UFFDIO_CONTINUE ioctl

On Wed, Feb 24, 2021 at 04:26:56PM -0800, Axel Rasmussen wrote:
> This ioctl is how userspace ought to resolve "minor" userfaults. The
> idea is, userspace is notified that a minor fault has occurred. It might
> change the contents of the page using its second non-UFFD mapping, or
> not. Then, it calls UFFDIO_CONTINUE to tell the kernel "I have ensured
> the page contents are correct, carry on setting up the mapping".
> 
> Note that it doesn't make much sense to use UFFDIO_{COPY,ZEROPAGE} for
> MINOR registered VMAs. ZEROPAGE maps the VMA to the zero page; but in
> the minor fault case, we already have some pre-existing underlying page.
> Likewise, UFFDIO_COPY isn't useful if we have a second non-UFFD mapping.
> We'd just use memcpy() or similar instead.
> 
> It turns out hugetlb_mcopy_atomic_pte() already does very close to what
> we want, if an existing page is provided via `struct page **pagep`. We
> already special-case the behavior a bit for the UFFDIO_ZEROPAGE case, so
> just extend that design: add an enum for the three modes of operation,
> and make the small adjustments needed for the MCOPY_ATOMIC_CONTINUE
> case. (Basically, look up the existing page, and avoid adding the
> existing page to the page cache or calling set_page_huge_active() on
> it.)
> 
> Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>

Reviewed-by: Peter Xu <peterx@...hat.com>

-- 
Peter Xu