Michal Hocko <mhocko@xxxxxxx> writes:
> On Wed 13-06-12 16:59:23, Michal Hocko wrote:
>> On Wed 13-06-12 15:57:23, Aneesh Kumar K.V wrote:
>> > From: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
>> >
>> > Use a mmu_gather instead of a temporary linked list for accumulating
>> > pages when we unmap a hugepage range
>>
>> Sorry for coming up with the comment that late but you owe us an
>> explanation _why_ you are doing this.
>>
>> I assume that this fixes a real problem when we take i_mmap_mutex
>> already up in
>> unmap_mapping_range
>> mutex_lock(&mapping->i_mmap_mutex);
>> unmap_mapping_range_tree | unmap_mapping_range_list
>> unmap_mapping_range_vma
>> zap_page_range_single
>> unmap_single_vma
>> unmap_hugepage_range
>> mutex_lock(&vma->vm_file->f_mapping->i_mmap_mutex);
>>
>> And that this should have been marked for stable as well (I haven't
>> checked when this has been introduced).
>>
>> But then I do not see how this help when you still do this:
>> [...]
>> > diff --git a/mm/memory.c b/mm/memory.c
>> > index 1b7dc66..545e18a 100644
>> > --- a/mm/memory.c
>> > +++ b/mm/memory.c
>> > @@ -1326,8 +1326,11 @@ static void unmap_single_vma(struct mmu_gather *tlb,
>> > * Since no pte has actually been setup, it is
>> > * safe to do nothing in this case.
>> > */
>> > - if (vma->vm_file)
>> > - unmap_hugepage_range(vma, start, end, NULL);
>> > + if (vma->vm_file) {
>> > + mutex_lock(&vma->vm_file->f_mapping->i_mmap_mutex);
>> > + __unmap_hugepage_range(tlb, vma, start, end, NULL);
>> > + mutex_unlock(&vma->vm_file->f_mapping->i_mmap_mutex);
>> > + }
>> > } else
>> > unmap_page_range(tlb, vma, start, end, details);
>> > }
>
> Ahhh, you are removing the lock in the next patch. Really confusing and
> not nice for the stable backport.
> Could you merge those two patches and add Cc: stable?
> Then you can add my
> Reviewed-by: Michal Hocko <mhocko@xxxxxxx>
>
In the last review cycle I was asked to see if we can get a lockdep
report for the above and what I found was we don't really cause the
above deadlock with the current codebase because for hugetlb we don't
directly call unmap_mapping_range. But still it is good to remove the
i_mmap_mutex, because we don't need that protection now. I didn't
mark it for stable because of the above reason.
-aneesh
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