Messages in this thread | | | Date | Wed, 19 Feb 2020 12:39:55 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH v6 1/2] mm: Add MREMAP_DONTUNMAP to mremap(). |
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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?
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