Re: [RFC PATCH 1/1] mm/mremap: add MREMAP_MIRROR flag for existing mirroring functionality
From: Kirill A. Shutemov
Date: Fri Jul 07 2017 - 13:46:13 EST
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
Kirill A. Shutemov