linux-kernel - Re: [PATCH] mm: Add MREMAP

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Message-ID: <CADyq12zvTnCtkBjDZjj_dQHapFYuTh=W=KXCEa9Hy5M9DXpugg@mail.gmail.com>
Date:   Thu, 23 Jan 2020 11:03:44 -0800
From:   Brian Geffon <bgeffon@...gle.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, Sonny Rao <sonnyrao@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-api@...r.kernel.org,
        Yu Zhao <yuzhao@...gle.com>, Jesse Barnes <jsbarnes@...gle.com>
Subject: Re: [PATCH] mm: Add MREMAP_DONTUNMAP to mremap().

Andy,

Thanks, yes, that's a much clearer description of the feature. I'll
make sure to update the description with subsequent patches and with
later man page updates.

Brian

On Wed, Jan 22, 2020 at 7:02 PM Andy Lutomirski <luto@...capital.net> wrote:
>
>
>
> > On Jan 22, 2020, at 5:46 PM, Brian Geffon <bgeffon@...gle.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.