Re: [PATCH -mm] mm, hugetlb: Pass fault address to no page handler

[Michal Hocko]

On Tue 15-05-18 08:57:56, Huang, Ying wrote:
> From: Huang Ying <ying.huang@xxxxxxxxx>
> 
> This is to take better advantage of huge page clearing
> optimization (c79b57e462b5d, "mm: hugetlb: clear target sub-page last
> when clearing huge page").  Which will clear to access sub-page last
> to avoid the cache lines of to access sub-page to be evicted when
> clearing other sub-pages.  This needs to get the address of the
> sub-page to access, that is, the fault address inside of the huge
> page.  So the hugetlb no page fault handler is changed to pass that
> information.  This will benefit workloads which don't access the begin
> of the huge page after page fault.
> 
> With this patch, the throughput increases ~28.1% in vm-scalability
> anon-w-seq test case with 88 processes on a 2 socket Xeon E5 2699 v4
> system (44 cores, 88 threads).  The test case creates 88 processes,
> each process mmap a big anonymous memory area and writes to it from
> the end to the begin.  For each process, other processes could be seen
> as other workload which generates heavy cache pressure.  At the same
> time, the cache miss rate reduced from ~36.3% to ~25.6%, the
> IPC (instruction per cycle) increased from 0.3 to 0.37, and the time
> spent in user space is reduced ~19.3%

This paragraph is confusing as Mike mentioned already. It would be
probably more helpful to see how was the test configured to use hugetlb
pages and what is the end benefit.

I do not have any real objection to the implementation so feel free to
add
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
I am just wondering what is the usecase driving this. Or is it just a
generic optimization that always makes sense to do? Indicating that in
the changelog would be helpful as well.

Thanks!

> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Andi Kleen <andi.kleen@xxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Shaohua Li <shli@xxxxxx>
> Cc: Christopher Lameter <cl@xxxxxxxxx>
> Cc: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> Cc: Punit Agrawal <punit.agrawal@xxxxxxx>
> Cc: Anshuman Khandual <khandual@xxxxxxxxxxxxxxxxxx>
> ---
>  mm/hugetlb.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 129088710510..3de6326abf39 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -3677,7 +3677,7 @@ int huge_add_to_page_cache(struct page *page, struct address_space *mapping,
>  
>  static int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  			   struct address_space *mapping, pgoff_t idx,
> -			   unsigned long address, pte_t *ptep, unsigned int flags)
> +			   unsigned long faddress, pte_t *ptep, unsigned int flags)
>  {
>  	struct hstate *h = hstate_vma(vma);
>  	int ret = VM_FAULT_SIGBUS;
> @@ -3686,6 +3686,7 @@ static int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  	struct page *page;
>  	pte_t new_pte;
>  	spinlock_t *ptl;
> +	unsigned long address = faddress & huge_page_mask(h);
>  
>  	/*
>  	 * Currently, we are forced to kill the process in the event the
> @@ -3749,7 +3750,7 @@ static int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  				ret = VM_FAULT_SIGBUS;
>  			goto out;
>  		}
> -		clear_huge_page(page, address, pages_per_huge_page(h));
> +		clear_huge_page(page, faddress, pages_per_huge_page(h));
>  		__SetPageUptodate(page);
>  		set_page_huge_active(page);
>  
> @@ -3871,7 +3872,7 @@ u32 hugetlb_fault_mutex_hash(struct hstate *h, struct mm_struct *mm,
>  #endif
>  
>  int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> -			unsigned long address, unsigned int flags)
> +			unsigned long faddress, unsigned int flags)
>  {
>  	pte_t *ptep, entry;
>  	spinlock_t *ptl;
> @@ -3883,8 +3884,7 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>  	struct hstate *h = hstate_vma(vma);
>  	struct address_space *mapping;
>  	int need_wait_lock = 0;
> -
> -	address &= huge_page_mask(h);
> +	unsigned long address = faddress & huge_page_mask(h);
>  
>  	ptep = huge_pte_offset(mm, address, huge_page_size(h));
>  	if (ptep) {
> @@ -3914,7 +3914,7 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>  
>  	entry = huge_ptep_get(ptep);
>  	if (huge_pte_none(entry)) {
> -		ret = hugetlb_no_page(mm, vma, mapping, idx, address, ptep, flags);
> +		ret = hugetlb_no_page(mm, vma, mapping, idx, faddress, ptep, flags);
>  		goto out_mutex;
>  	}
>  
> -- 
> 2.16.1
> 

-- 
Michal Hocko
SUSE Labs