Messages in this thread |  | | | Date | Wed, 07 Apr 2010 11:52:59 -0400 | | From | Rik van Riel <> | | Subject | Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon_vmas of a mergeable VMA |
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On 04/07/2010 11:30 AM, Linus Torvalds wrote:
> I've said this now _three_ times, but let me repeat once more: > > - the locking rules for that anon_vma_chain are very unclear. I _think_ > you mean for them to be "mmap_sem held for writing, _or_ mmap_sem held > for reading and page_table_lock held", but nowhere is that actually > documented.
> Why is it so hard for you to just admit that? Especially after you > yourself got it wrong.
You are right, the idea was to continue use the locking that the anon_vma code was already using, without introducing any new locking with the anon_vma patches.
However, it has become clear that this is no longer possible, due to the need to hold a secondary lock across anon_vma_clone, when we come from a code path that holds the mmap_sem for read.
>> + merge_vma = find_mergeable_anon_vma(vma); >> + if (merge_vma) { >> + int ret; >> + spin_lock(&mm->page_table_lock); >> + ret = anon_vma_clone(vma, merge_vma); >> + if (!ret) >> + vma->anon_vma = merge_vma->anon_vma; >> + spin_unlock(&mm->page_table_lock); >> + return ret; >> + } > > Rik, the above is obviously total crap. > > anon_vma_clone() needs to allocate memory, and it does so with GFP_KERNEL. > You can't do that with a spinlock held.
Looks like we'll either have to introduce a per-mm semaphore for the same_vma anon_vma chains, or move the complexity of solving this bug to anon_vma_merge, where we can ensure that the resulting VMA has the sum of the anon_vmas of each VMA.
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