From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Kravetz Subject: [RFC PATCH 0/1] mm/mremap: add MREMAP_MIRROR flag Date: Thu, 6 Jul 2017 09:17:25 -0700 Message-ID: <1499357846-7481-1-git-send-email-mike.kravetz@oracle.com> Return-path: Sender: owner-linux-mm@kvack.org To: linux-mm@kvack.org, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org Cc: Andrew Morton , Andrea Arcangeli , Michal Hocko , Aaron Lu , "Kirill A . Shutemov" , Mike Kravetz List-Id: linux-api@vger.kernel.org The mremap system call has the ability to 'mirror' parts of an existing mapping. To do so, it creates a new mapping that maps the same pages as the original mapping, just at a different virtual address. This functionality has existed since at least the 2.6 kernel [1]. A comment was added to the code to help preserve this feature. The Oracle JVM team has discovered this feature and used it while prototyping a new garbage collection model. This new model shows promise, and they are considering its use in a future release. However, since the only mention of this functionality is a single comment in the kernel, they are concerned about its future. I propose the addition of a new MREMAP_MIRROR flag to explicitly request this functionality. The flag simply provides the same functionality as the existing undocumented 'old_size == 0' interface. As an alternative, we could simply document the 'old_size == 0' interface in the man page. In either case, man page modifications would be needed. Future Direction After more formally adding this to the API (either new flag or documenting existing interface), the mremap code could be enhanced to optimize this case. Currently, 'mirroring' only sets up the new mapping. It does not create page table entries for new mapping. This could be added as an enhancement. The JVM today has the option of using (static) huge pages. The mremap system call does not fully support huge page mappings today. You can use mremap to shrink the size of a huge page mapping, but it can not be used to expand or mirror a mapping. Such support is fairly straight forward. [1] https://lkml.org/lkml/2004/1/12/260 Mike Kravetz (1): mm/mremap: add MREMAP_MIRROR flag for existing mirroring functionality include/uapi/linux/mman.h | 5 +++-- mm/mremap.c | 23 ++++++++++++++++------- tools/include/uapi/linux/mman.h | 5 +++-- 3 files changed, 22 insertions(+), 11 deletions(-) -- 2.7.5 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org