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Date:   Mon, 24 Feb 2020 11:16:22 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Brian Geffon <bgeffon@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-api@vger.kernel.org,
        Andy Lutomirski <luto@amacapital.net>,
        Will Deacon <will@kernel.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Sonny Rao <sonnyrao@google.com>,
        Minchan Kim <minchan@kernel.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Yu Zhao <yuzhao@google.com>,
        Jesse Barnes <jsbarnes@google.com>,
        Florian Weimer <fweimer@redhat.com>, mtk.manpages@gmail.com,
        linux-man@vger.kernel.org, Lokesh Gidra <lokeshgidra@google.com>
Subject: Re: [PATCH v7 1/2] mm: Add MREMAP_DONTUNMAP to mremap().
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On Fri, Feb 21, 2020 at 09:42:46AM -0800, Brian Geffon wrote:
> When remapping an anonymous, private mapping, if MREMAP_DONTUNMAP is
> set, the source mapping will not be removed. The remap operation
> will be performed as it would have been normally by moving over the
> page tables to the new mapping. The old vma will have any locked
> flags cleared, have no pagetables, and any userfaultfds that were
> watching that range will continue watching it.
> 
> For a mapping that is shared or not anonymous, MREMAP_DONTUNMAP will cause
> the mremap() call to fail. Because MREMAP_DONTUNMAP always results in moving
> a VMA you MUST use the MREMAP_MAYMOVE flag, it's not possible to resize
> a VMA while also moving with MREMAP_DONTUNMAP so old_len must always
> be equal to the new_len otherwise it will return -EINVAL.
> 
> We hope to use this in Chrome OS where with userfaultfd we could write
> an anonymous mapping to disk without having to STOP the process or worry
> about VMA permission changes.
> 
> This feature also has a use case in Android, Lokesh Gidra has said
> that "As part of using userfaultfd for GC, We'll have to move the physical
> pages of the java heap to a separate location. For this purpose mremap
> will be used. Without the MREMAP_DONTUNMAP flag, when I mremap the java
> heap, its virtual mapping will be removed as well. Therefore, we'll
> require performing mmap immediately after. This is not only time consuming
> but also opens a time window where a native thread may call mmap and
> reserve the java heap's address range for its own usage. This flag
> solves the problem."
> 
>   v6 -> v7:
>     - Don't allow resizing VMA as part of MREMAP_DONTUNMAP.
>       There is no clear use case at the moment and it can be added
>       later as it simplifies the implementation for now.
> 
>   v5 -> v6:
>     - Code cleanup suggested by Kirill.
> 
>   v4 -> v5:
>     - Correct commit message to more accurately reflect the behavior.
>     - Clear VM_LOCKED and VM_LOCKEDONFAULT on the old vma.
>         ? ?
> Signed-off-by: Brian Geffon <bgeffon@google.com>
> Reviewed-by: Minchan Kim <minchan@kernel.org>
> Tested-by: Lokesh Gidra <lokeshgidra@google.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

-- 
 Kirill A. Shutemov