From: Andy Lutomirski <luto@amacapital.net>
To: Brian Geffon <bgeffon@google.com>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
"Michael S . Tsirkin" <mst@redhat.com>,
Arnd Bergmann <arnd@arndb.de>, Sonny Rao <sonnyrao@google.com>,
Minchan Kim <minchan@kernel.org>,
Joel Fernandes <joel@joelfernandes.org>,
Lokesh Gidra <lokeshgidra@google.com>,
linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
Yu Zhao <yuzhao@google.com>, Jesse Barnes <jsbarnes@google.com>
Subject: Re: [PATCH] mm: Add MREMAP_DONTUNMAP to mremap().
Date: Wed, 22 Jan 2020 19:02:08 -0800 [thread overview]
Message-ID: <CD5EA896-7364-40E2-8709-A014973FFBC8@amacapital.net> (raw)
In-Reply-To: <20200123014627.71720-1-bgeffon@google.com>
> On Jan 22, 2020, at 5:46 PM, Brian Geffon <bgeffon@google.com> wrote:
>
> MREMAP_DONTUNMAP is an additional flag that can be used with
> MREMAP_FIXED to move a mapping to a new address. Normally, mremap(2)
> would then tear down the old vma so subsequent accesses to the vma
> cause a segfault. However, with this new flag it will keep the old
> vma with zapping PTEs so any access to the old VMA after that point
> will result in a pagefault.
This needs a vastly better description. Perhaps:
When remapping an anonymous, private mapping, if MREMAP_DONTUNMAP is set, the source mapping will not be removed. Instead it will be cleared as if a brand new anonymous, private mapping had been created atomically as part of the mremap() call. If a userfaultfd was watching the source, it will continue to watch the new mapping. For a mapping that is shared or not anonymous, MREMAP_DONTUNMAP will cause the mremap() call to fail.
Or is it something else?
>
> This feature will find a use in ChromeOS along with userfaultfd.
> Specifically we will want to register a VMA with userfaultfd and then
> pull it out from under a running process. By using MREMAP_DONTUNMAP we
> don't have to worry about mprotecting and then potentially racing with
> VMA permission changes from a running process.
Does this mean you yank it out but you want to replace it simultaneously?
>
> 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."
Cute.
next prev parent reply other threads:[~2020-01-23 3:02 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-23 1:46 [PATCH] mm: Add MREMAP_DONTUNMAP to mremap() Brian Geffon
2020-01-23 3:02 ` Andy Lutomirski [this message]
2020-01-23 19:03 ` Brian Geffon
2020-01-24 19:06 ` [PATCH v2] " Brian Geffon
2020-01-26 5:16 ` Nathan Chancellor
2020-01-27 2:21 ` Brian Geffon
2020-01-26 22:06 ` Kirill A. Shutemov
2020-01-28 1:35 ` Brian Geffon
2020-01-29 10:46 ` Kirill A. Shutemov
2020-02-01 21:03 ` Brian Geffon
2020-02-02 4:17 ` Brian Geffon
2020-02-03 13:09 ` Kirill A. Shutemov
2020-02-07 20:42 ` Brian Geffon
2020-02-10 10:35 ` Kirill A. Shutemov
2020-01-27 10:13 ` Florian Weimer
2020-01-27 22:33 ` Brian Geffon
2020-01-30 12:19 ` Florian Weimer
2020-01-27 4:46 ` [PATCH] " Dan Carpenter
2020-01-27 5:30 ` [PATCH v3] " Brian Geffon
2020-01-28 15:26 ` Will Deacon
2020-01-30 10:12 ` Will Deacon
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