From: Andrew Morton <akpm@linux-foundation.org>
To: Brian Geffon <bgeffon@google.com>
Cc: "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>,
"Kirill A . Shutemov" <kirill@shutemov.name>,
lokeshgidra@google.com
Subject: Re: [PATCH v6 1/2] mm: Add MREMAP_DONTUNMAP to mremap().
Date: Wed, 19 Feb 2020 12:39:55 -0800 [thread overview]
Message-ID: <20200219123955.dc97c43524d6e6ab92722650@linux-foundation.org> (raw)
In-Reply-To: <20200218173221.237674-1-bgeffon@google.com>
On Tue, 18 Feb 2020 09:32:20 -0800 Brian Geffon <bgeffon@google.com> 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. The final result is two
> equally sized VMAs where the destination contains the PTEs of the source.
>
> 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."
Thanks.
We're a bit thin on review activity on this one. Has Lokesh been able
to review and preferably test the code? Are you able to identify other
potential users? perhaps even glibc?
next prev parent reply other threads:[~2020-02-19 20:39 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-18 17:32 [PATCH v6 1/2] mm: Add MREMAP_DONTUNMAP to mremap() Brian Geffon
2020-02-18 17:32 ` [PATCH v6 2/2] selftest: Add MREMAP_DONTUNMAP selftest Brian Geffon
2020-02-19 20:39 ` Andrew Morton [this message]
2020-02-19 21:38 ` [PATCH v6 1/2] mm: Add MREMAP_DONTUNMAP to mremap() Lokesh Gidra
2020-02-19 23:23 ` Minchan Kim
2020-02-20 11:57 ` Kirill A. Shutemov
2020-02-20 23:55 ` Brian Geffon
2020-02-21 12:23 ` Kirill A. Shutemov
2020-02-20 17:15 ` Minchan Kim
2020-02-20 18:36 ` Brian Geffon
2020-02-20 18:45 ` Brian Geffon
2020-02-20 19:14 ` Minchan Kim
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