Subject: Re: [PATCH 03/10] mm/hugetlb: Protect follow_huge_(pud|pgd) functions from race

From: Anshuman Khandual <[email-masked]>
Date: Mon, 11 Apr 2016 11:09:35 +0530

On 04/07/2016 02:56 PM, Balbir Singh wrote:
> 
> On 07/04/16 15:37, Anshuman Khandual wrote:
>> > follow_huge_(pmd|pud|pgd) functions are used to walk the page table and
>> > fetch the page struct during 'follow_page_mask' call. There are possible
>> > race conditions faced by these functions which arise out of simultaneous
>> > calls of move_pages() and freeing of huge pages. This was fixed partly
>> > by the previous commit e66f17ff7177 ("mm/hugetlb: take page table lock
>> > in follow_huge_pmd()") for only PMD based huge pages.
>> > 
>> > After implementing similar logic, functions like follow_huge_(pud|pgd)
>> > are now safe from above mentioned race conditions and also can support
>> > FOLL_GET. Generic version of the function 'follow_huge_addr' has been
>> > left as it is and its upto the architecture to decide on it.
>> > 
>> > Signed-off-by: Anshuman Khandual <[email-masked]>
>> > ---
>> >  include/linux/mm.h | 33 +++++++++++++++++++++++++++
>> >  mm/hugetlb.c       | 67 ++++++++++++++++++++++++++++++++++++++++++++++--------
>> >  2 files changed, 91 insertions(+), 9 deletions(-)
>> > 
>> > diff --git a/include/linux/mm.h b/include/linux/mm.h
>> > index ffcff53..734182a 100644
>> > --- a/include/linux/mm.h
>> > +++ b/include/linux/mm.h
>> > @@ -1751,6 +1751,19 @@ static inline void pgtable_page_dtor(struct page *page)
>> >  		NULL: pte_offset_kernel(pmd, address))
>> >  
>> >  #if USE_SPLIT_PMD_PTLOCKS
> Do we still use USE_SPLIT_PMD_PTLOCKS? I think its good enough. with pgd's
> we are likely to use the same locks and the split nature may not be really
> split.
> 

Sorry Balbir, did not get what you asked. Can you please elaborate on
this ?