Re: [RESEND RFC v4 3/3] mm: make swapin readahead to improve thp collapse rate

[Vlastimil Babka]

On 09/03/2015 10:51 PM, Ebru Akagunduz wrote:
> This patch makes swapin readahead to improve thp collapse rate.
> When khugepaged scanned pages, there can be a few of the pages
> in swap area.
> 
> With the patch THP can collapse 4kB pages into a THP when
> there are up to max_ptes_swap swap ptes in a 2MB range.
> 
> The patch was tested with a test program that allocates
> 800MB of memory, writes to it, and then sleeps. I force
> the system to swap out all. Afterwards, the test program
> touches the area by writing, it skips a page in each
> 20 pages of the area.
> 
> Without the patch, system did not swap in readahead.
> THP rate was %47 of the program of the memory, it
> did not change over time.
> 
> With this patch, after 10 minutes of waiting khugepaged had
> collapsed %90 of the program's memory.
> 
> Signed-off-by: Ebru Akagunduz <ebru.akagunduz@xxxxxxxxx>
> Acked-by: Rik van Riel <riel@xxxxxxxxxx>
> ---
> Changes in v2:
>  - Use FAULT_FLAG_ALLOW_RETRY|FAULT_FLAG_RETRY_NOWAIT flag
>    instead of 0x0 when called do_swap_page from
>    __collapse_huge_page_swapin (Rik van Riel)
> 
> Changes in v3:
>  - Catch VM_FAULT_HWPOISON and VM_FAULT_OOM return cases
>    in __collapse_huge_page_swapin (Kirill A. Shutemov)
> 
> Changes in v4:
>  - Fix broken indentation reverting if (...) statement in
>    __collapse_huge_page_swapin (Kirill A. Shutemov)
>  - Fix check statement of ret (Kirill A. Shutemov)
>  - Use swapped_in name instead of swap_pte
> 
> 			After swapped out
> -------------------------------------------------------------------
>               | Anonymous | AnonHugePages | Swap      | Fraction  |
> -------------------------------------------------------------------
> With patch    | 253720 kB | 251904 kB     | 546284 kB |    %99    |
> -------------------------------------------------------------------
> Without patch | 238160 kB | 235520 kB     | 561844 kB |    %98    |
> -------------------------------------------------------------------
> 
>                         After swapped in
> -------------------------------------------------------------------
>               | Anonymous | AnonHugePages | Swap      | Fraction  |
> -------------------------------------------------------------------
> With patch    | 533532 kB | 528384 kB     | 266472 kB |    %90    |
> -------------------------------------------------------------------
> Without patch | 499956 kB | 235520 kB     | 300048 kB |    %47    |
> -------------------------------------------------------------------
> 
> 
>  include/linux/mm.h                 |  4 ++++
>  include/trace/events/huge_memory.h | 24 +++++++++++++++++++++
>  mm/huge_memory.c                   | 43 ++++++++++++++++++++++++++++++++++++++
>  mm/memory.c                        |  2 +-
>  4 files changed, 72 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index bf6f117..f28d98a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -29,6 +29,10 @@ struct user_struct;
>  struct writeback_control;
>  struct bdi_writeback;
>  
> +extern int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
> +			unsigned long address, pte_t *page_table, pmd_t *pmd,
> +			unsigned int flags, pte_t orig_pte);
> +

I'd rather use mm/internal.h here than export the function outside mm.

>  #ifndef CONFIG_NEED_MULTIPLE_NODES	/* Don't use mapnrs, do it properly */
>  extern unsigned long max_mapnr;
>  
> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> index c2112fd..e530210 100644
> --- a/include/trace/events/huge_memory.h
> +++ b/include/trace/events/huge_memory.h
> @@ -97,6 +97,30 @@ TRACE_EVENT(mm_collapse_huge_page_isolate,
>  		khugepaged_status_string[__entry->status])
>  );
>  
> +TRACE_EVENT(mm_collapse_huge_page_swapin,
> +
> +	TP_PROTO(struct mm_struct *mm, int swapped_in, int ret),
> +
> +	TP_ARGS(mm, swapped_in, ret),
> +
> +	TP_STRUCT__entry(
> +		__field(struct mm_struct *, mm)
> +		__field(int, swapped_in)
> +		__field(int, ret)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->mm = mm;
> +		__entry->swapped_in = swapped_in;
> +		__entry->ret = ret;
> +	),
> +
> +	TP_printk("mm=%p, swapped_in=%d, ret=%d",
> +		__entry->mm,
> +		__entry->swapped_in,
> +		__entry->ret)
> +);
> +
>  #endif /* __HUGE_MEMORY_H */
>  #include <trace/define_trace.h>
>  
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index cb9ab46..f420ef8 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2529,6 +2529,47 @@ static bool hugepage_vma_check(struct vm_area_struct *vma)
>  	return true;
>  }
>  
> +/*
> + * Bring missing pages in from swap, to complete THP collapse.
> + * Only done if khugepaged_scan_pmd believes it is worthwhile.
> + *
> + * Called and returns without pte mapped or spinlocks held,
> + * but with mmap_sem held to protect against vma changes.
> + */
> +
> +static void __collapse_huge_page_swapin(struct mm_struct *mm,
> +					struct vm_area_struct *vma,
> +					unsigned long address, pmd_t *pmd,
> +					pte_t *pte)
> +{
> +	unsigned long _address;
> +	pte_t pteval = *pte;
> +	int swapped_in = 0, ret = 0;
> +
> +	pte = pte_offset_map(pmd, address);
> +	for (_address = address; _address < address + HPAGE_PMD_NR*PAGE_SIZE;
> +	     pte++, _address += PAGE_SIZE) {
> +		pteval = *pte;
> +		if (!is_swap_pte(pteval))
> +			continue;
> +		swapped_in++;
> +		ret = do_swap_page(mm, vma, _address, pte, pmd,
> +				   FAULT_FLAG_ALLOW_RETRY|FAULT_FLAG_RETRY_NOWAIT,
> +				   pteval);
> +		if (ret & VM_FAULT_ERROR) {
> +			trace_mm_collapse_huge_page_swapin(mm, swapped_in, 0);
> +			return;
> +		}
> +		/* pte is unmapped now, we need to map it */
> +		pte = pte_offset_map(pmd, _address);
> +	}
> +	pte--;
> +	pte_unmap(pte);
> +	trace_mm_collapse_huge_page_swapin(mm, swapped_in, 1);
> +}
> +
> +
> +
>  static void collapse_huge_page(struct mm_struct *mm,
>  				   unsigned long address,
>  				   struct page **hpage,
> @@ -2600,6 +2641,8 @@ static void collapse_huge_page(struct mm_struct *mm,
>  
>  	anon_vma_lock_write(vma->anon_vma);
>  
> +	__collapse_huge_page_swapin(mm, vma, address, pmd, pte);

Hmm it seems rather wasteful to call this when no swap entries were detected.
Also it seems pointless to try continue collapsing when we have just only issued
async swap-in? What are the chances they would finish in time?

I'm less sure about the relation vs khugepaged_alloc_page(). At this point, we
have already succeeded the hugepage allocation. It makes sense not to swap-in if
we can't allocate a hugepage. It makes also sense not to allocate a hugepage if
we will just issue async swap-ins and then free the hugepage back. Swap-in means
disk I/O that's best avoided if not useful. But the reclaim for hugepage
allocation might also involve disk I/O. At worst, it could be creating new swap
pte's in the very pmd we are scanning... Thoughts?

> +
>  	pte = pte_offset_map(pmd, address);
>  	pte_ptl = pte_lockptr(mm, pmd);
>  
> diff --git a/mm/memory.c b/mm/memory.c
> index 388dcf9..d3a446c 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2442,7 +2442,7 @@ EXPORT_SYMBOL(unmap_mapping_range);
>   * We return with the mmap_sem locked or unlocked in the same cases
>   * as does filemap_fault().
>   */
> -static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
> +int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  		unsigned long address, pte_t *page_table, pmd_t *pmd,
>  		unsigned int flags, pte_t orig_pte)
>  {
> 

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