On 2016/7/26 15:58, Michal Hocko wrote:
> On Fri 22-07-16 07:17:37, Naoya Horiguchi wrote:
> [...]
>> I think that (src_pte != dst_pte) can happen and that's ok if there's no
>> migration entry.
> We have discussed that with Naoya off-list and couldn't find a scenario
> when parent would have !shared pmd while child would have it. The only
> plausible scenario was that parent created and poppulated mapping smaller
> than 1G and then enlarged it later on so the child would see sharedable
> pud. This doesn't seem to be possible because vma_merge would bail out
> due to VM_SPECIAL check.
I do not understand that the process must have vm_special flags. if vm_special enable,
the process must not be expanded. and what does it matter about vma_merge ??
>> But even if we have both of normal entry and migration entry
>> for one hugepage, that still looks fine to me because the running migration
>> operation fails (because there remains mapcounts on the source hugepage),
>> and all migration entries are turned back to normal entries pointing to the
>> source hugepage.
In one case, try_to_unmap_one is first exec and successfully, mapcount turn into zero.
then we get the pte lock, if src_pte!-dst_pte, it maybe lead to the dst_pte is from migrate pte
to normal pte, while the normal pte turn into migaret pte,, is right ?
>
> Agreed.
>
>> Could you try to see and share what happens on your workload with
>> Michal's patch?
> Zhong Jiang did you have chance to retest with the BUG_ON changed?
>
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