Re: [PATCH v6 1/2] mm: Add MREMAP_DONTUNMAP to mremap().

From: Minchan Kim <minchan@kernel.org>
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>,
	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>
Subject: Re: [PATCH v6 1/2] mm: Add MREMAP_DONTUNMAP to mremap().
Date: Wed, 19 Feb 2020 15:23:09 -0800	[thread overview]
Message-ID: <20200219232309.GB148976@google.com> (raw)
In-Reply-To: <20200218173221.237674-1-bgeffon@google.com>

On Tue, Feb 18, 2020 at 09:32:20AM -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. 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."
> 
>   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>

Description and implemenation looks clean/sane for me.

Reviewed-by: Minchan Kim <minchan@kernel.org>

When I review the patch, it seems to be broken with lots of "=20", not
sure it's my mail client problem or yours. Anyway, please double check
it.

Thanks.