mm/uffd: allow vma to merge as much as possible

We used to not pass in the pgoff correctly when register/unregister uffd
regions, it caused incorrect behavior on vma merging and can cause
mergeable vmas being separate after ioctls return.

For example, when we have:

  vma1(range 0-9, with uffd), vma2(range 10-19, no uffd)

Then someone unregisters uffd on range (5-9), it should logically become:

  vma1(range 0-4, with uffd), vma2(range 5-19, no uffd)

But with current code we'll have:

  vma1(range 0-4, with uffd), vma3(range 5-9, no uffd), vma2(range 10-19, no uffd)

This patch allows such merge to happen correctly before ioctl returns.

This behavior seems to have existed since the 1st day of uffd.  Since
pgoff for vma_merge() is only used to identify the possibility of vma
merging, meanwhile here what we did was always passing in a pgoff smaller
than what we should, so there should have no other side effect besides not
merging it.  Let's still tentatively copy stable for this, even though I
don't see anything will go wrong besides vma being split (which is mostly
not user visible).

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization")
Signed-off-by: Peter Xu <[email protected]>
Reported-by: Lorenzo Stoakes <[email protected]>
Acked-by: Lorenzo Stoakes <[email protected]>
Reviewed-by: Liam R. Howlett <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Mike Rapoport (IBM) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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