linux-kernel - Re: [PATCH v5 1/3] mm: Extend MREMAP_DONTUNMAP to non-anonymous mappings

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Message-ID: <fef17cb0-ae0a-db3f-e847-3febd318a81a@gmail.com>
Date:   Tue, 23 Mar 2021 19:04:34 +0000
From:   Dmitry Safonov <0x7f454c46@...il.com>
To:     Brian Geffon <bgeffon@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Peter Xu <peterx@...hat.com>, Hugh Dickins <hughd@...gle.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andy Lutomirski <luto@...capital.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Dmitry Safonov <dima@...sta.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Alejandro Colomar <alx.manpages@...il.com>
Subject: Re: [PATCH v5 1/3] mm: Extend MREMAP_DONTUNMAP to non-anonymous
 mappings

On 3/23/21 6:25 PM, Brian Geffon wrote:
> Currently MREMAP_DONTUNMAP only accepts private anonymous mappings.
> This restriction was placed initially for simplicity and not because
> there exists a technical reason to do so.
> 
> This change will widen the support to include any mappings which are not
> VM_DONTEXPAND or VM_PFNMAP. The primary use case is to support
> MREMAP_DONTUNMAP on mappings which may have been created from a memfd.
> This change will result in mremap(MREMAP_DONTUNMAP) returning -EINVAL
> if VM_DONTEXPAND or VM_PFNMAP mappings are specified.
> 
> Lokesh Gidra who works on the Android JVM, provided an explanation of how
> such a feature will improve Android JVM garbage collection:
> "Android is developing a new garbage collector (GC), based on userfaultfd.
> The garbage collector will use userfaultfd (uffd) on the java heap during
> compaction. On accessing any uncompacted page, the application threads will
> find it missing, at which point the thread will create the compacted page
> and then use UFFDIO_COPY ioctl to get it mapped and then resume execution.
> Before starting this compaction, in a stop-the-world pause the heap will be
> mremap(MREMAP_DONTUNMAP) so that the java heap is ready to receive
> UFFD_EVENT_PAGEFAULT events after resuming execution.

Pretty interesting idea :-)

> 
> To speedup mremap operations, pagetable movement was optimized by moving
> PUD entries instead of PTE entries [1]. It was necessary as mremap of even
> modest sized memory ranges also took several milliseconds, and stopping the
> application for that long isn't acceptable in response-time sensitive
> cases.
> 
> With UFFDIO_CONTINUE feature [2], it will be even more efficient to
> implement this GC, particularly the 'non-moveable' portions of the heap.
> It will also help in reducing the need to copy (UFFDIO_COPY) the pages.
> However, for this to work, the java heap has to be on a 'shared' vma.
> Currently MREMAP_DONTUNMAP only supports private anonymous mappings, this
> patch will enable using UFFDIO_CONTINUE for the new userfaultfd-based heap
> compaction."
> 
> [1] https://lore.kernel.org/linux-mm/20201215030730.NC3CU98e4%25akpm@linux-foundation.org/
> [2] https://lore.kernel.org/linux-mm/20210302000133.272579-1-axelrasmussen@google.com/
> 
> Signed-off-by: Brian Geffon <bgeffon@...gle.com>
> Acked-by: Hugh Dickins <hughd@...gle.com>
> Tested-by: Lokesh Gidra <lokeshgidra@...gle.com>

Reviewed-by: Dmitry Safonov <0x7f454c46@...il.com>

Thanks,
Dmitry