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. -- Mike Kravetz
