Re: [PATCH] mremap: enforce rmap src/dst vma ordering in case of vma_merge succeeding in copy_vma

[Andrea Arcangeli <aarcange@redhat.com>]

On Wed, Nov 09, 2011 at 02:25:42AM +0100, Andrea Arcangeli wrote:
> Also note, if we find a way to enforce orderings in the prio tree (not
> sure if it's possible, apparently it's already using list_add_tail
> so..), then we could also remove the i_mmap_lock from mremap and fork.

I'm not optimistic we can enforce ordering there. Being a tree it's
walked in range order.

I thought of another solution that would avoid having to reorder the
list in mremap and avoid the i_mmap_mutex to be added to fork (and
then we can remove it from mremap too). The solution is to rmap_walk
twice. I mean two loops over the same_anon_vma for those rmap walks
that must be reliable (that includes two calls of
unmap_mapping_range). For both same_anon_vma and prio tree.

Reading truncate_pagecache I see two loops already and a comment
saying it's for fork(), to avoid leaking ptes in the child. So fork is
probably ok already without having to take the i_mmap_mutex, but then
I wonder why that also doesn't fix mremap if we do two loops there and
why that i_mmap_mutex is really needed in mremap considering those two
calls already present in truncate_pagecache. I wonder if that was a
"theoretical" fix that missed the fact truncate already walks the prio
tree twice, so it doesn't matter if the rmap_walk goes in the opposite
direction of move_page_tables? That i_mmap_lock in mremap (now
i_mmap_mutex) is there since start of git history. The double loop was
introduced in d00806b183152af6d24f46f0c33f14162ca1262a. So it's very
possible that i_mmap_mutex is now useless (after
d00806b183152af6d24f46f0c33f14162ca1262a) and the fix for fork, was
already taking care of mremap too and that i_mmap_mutex can now be
removed.