lkml.org 
[lkml]   [2020]   [Feb]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v6 1/2] mm: Add MREMAP_DONTUNMAP to mremap().
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.

\
 
 \ /
  Last update: 2020-02-20 00:24    [W:0.458 / U:0.324 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site