Messages in this thread | | | Date | Wed, 16 Nov 2011 15:00:42 +0100 | From | Andrea Arcangeli <> | Subject | Re: [PATCH] mremap: enforce rmap src/dst vma ordering in case of vma_merge succeeding in copy_vma |
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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.
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