On 07/07/2017 01:19 AM, Anshuman Khandual wrote: > On 07/06/2017 09:47 PM, Mike Kravetz wrote: >> 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. > > > Is this the comment ? If yes, then its not very clear. > > /* > * We allow a zero old-len as a special case > * for DOS-emu "duplicate shm area" thing. But > * a zero new-len is nonsensical. > */ > Yes, I believe that is the comment. >> >> 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. > > Right. Adding MREMAP_MIRROR sounds cleaner from application programming > point of view. But it extends the interface. Yes. That is the reason for the RFC. We currently have functionality that is not clearly part of a programming interface. Application programmers do not like to depend on something that is not part of an interface. >> >> 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. > > Then how it achieves mirroring, both the pointers should see the same > data, that can happen with page table entries pointing to same pages, > right ? Correct. In the code today, page tables for the new (mirrored) mapping are created as needed via faults. The enhancement would be to create page table entries for the new mapping. -- Mike Kravetz -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
