On 07/07/2017 11:39 PM, Mike Kravetz wrote: > On 07/07/2017 10:45 AM, Kirill A. Shutemov wrote: >> On Fri, Jul 07, 2017 at 10:29:52AM -0700, Mike Kravetz wrote: >>> On 07/07/2017 03:23 AM, Kirill A. Shutemov wrote: >>>> On Thu, Jul 06, 2017 at 09:17:26AM -0700, 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. >>>>> >>>>> This patch simply adds a new flag to mremap which will make this >>>>> functionality part of the API. It maintains backward compatibility with >>>>> the existing way of requesting mirroring (old_size == 0). >>>>> >>>>> If this new MREMAP_MIRROR flag is specified, then new_size must equal >>>>> old_size. In addition, the MREMAP_MAYMOVE flag must be specified. >>>> >>>> The patch breaks important invariant that anon page can be mapped into a >>>> process only once. >>> >>> Actually, the patch does not add any new functionality. It only provides >>> a new interface to existing functionality. >>> >>> Is it not possible to have an anon page mapped twice into the same process >>> via system V shared memory? shmget(anon), shmat(), shmat. >>> Of course, those are shared rather than private anon pages. >> >> By anon pages I mean, private anon or file pages. These are subject to CoW. >> >>>> What is going to happen to mirrored after CoW for instance? >>>> >>>> In my opinion, it shouldn't be allowed for anon/private mappings at least. >>>> And with this limitation, I don't see much sense in the new interface -- >>>> just create mirror by mmap()ing the file again. >>> >>> The code today works for anon shared mappings. See simple program below. >>> >>> You are correct in that it makes little or no sense for private mappings. >>> When looking closer at existing code, mremap() creates a new private >>> mapping in this case. This is most likely a bug. >> >> IIRC, existing code doesn't create mirrors of private pages as it requires >> old_len to be zero. There's no way to get private pages mapped twice this >> way. > > Correct. > As mentioned above, mremap does 'something' for private anon pages when > old_len == 0. However, this may be considered a bug. In this case, mremap > creates a new private anon mapping of length new_size. Since old_len == 0, > it does not unmap any of the old mapping. So, in this case mremap basically > creates a new private mapping (unrealted to the original) and does not > modify the old mapping. > Yeah, in my experiment, after the mremap() exists we have two different VMAs which can contain two different set of data. No page sharing is happening. -- 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
