Re: [RFC v2 3/3] mm: make swapin readahead to improve thp collapse rate

Kirill A. Shutemov
2015-06-21T18:12:06Z

On Sat, Jun 20, 2015 at 02:28:06PM +0300, 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 %99 of the program's memory.
>
> Signed-off-by: Ebru Akagunduz <ebru.akagunduz@gmail.com>
> Acked-by: Rik van Riel <riel@redhat.com>
> ---
> 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
>
> Test results:
>
> After swapped out
> -------------------------------------------------------------------
> | Anonymous | AnonHugePages | Swap | Fraction |
> -------------------------------------------------------------------
> With patch | 265772 kB | 264192 kB | 534232 kB | %99 |
> -------------------------------------------------------------------
> Without patch | 238160 kB | 235520 kB | 561844 kB | %98 |
> -------------------------------------------------------------------
>
> After swapped in
> -------------------------------------------------------------------
> | Anonymous | AnonHugePages | Swap | Fraction |
> -------------------------------------------------------------------
> With patch | 532756 kB | 528384 kB | 267248 kB | %99 |
> -------------------------------------------------------------------
> 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 | 37 +++++++++++++++++++++++++++++++++++++
> mm/memory.c | 2 +-
> 4 files changed, 66 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 7f47178..f66ff8a 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);
> +
> #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 53c9f2e..0117ab9 100644
> --- a/include/trace/events/huge_memory.h
> +++ b/include/trace/events/huge_memory.h
> @@ -95,5 +95,29 @@ TRACE_EVENT(mm_collapse_huge_page_isolate,
> __entry->writable)
> );
>
> +TRACE_EVENT(mm_collapse_huge_page_swapin,
> +
> + TP_PROTO(struct mm_struct *mm, unsigned long vm_start, int swap_pte),
> +
> + TP_ARGS(mm, vm_start, swap_pte),
> +
> + TP_STRUCT__entry(
> + __field(struct mm_struct *, mm)
> + __field(unsigned long, vm_start)
> + __field(int, swap_pte)
> + ),
> +
> + TP_fast_assign(
> + __entry->mm = mm;
> + __entry->vm_start = vm_start;
> + __entry->swap_pte = swap_pte;
> + ),
> +
> + TP_printk("mm=%p, vm_start=%04lx, swap_pte=%d",
> + __entry->mm,
> + __entry->vm_start,
> + __entry->swap_pte)
> +);
> +
> #endif /* __HUGE_MEMORY_H */
> #include <trace/define_trace.h>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 22bc0bf..064fd72 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2496,6 +2496,41 @@ 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 swap_pte = 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)) {
> + swap_pte++;
> + do_swap_page(mm, vma, _address, pte, pmd,
> + FAULT_FLAG_ALLOW_RETRY|FAULT_FLAG_RETRY_NOWAIT,
> + pteval);

Hm. I guess this lacking error handling.
We really should abort early at least for VM_FAULT_HWPOISON and VM_FAULT_OOM.

> + /* pte is unmapped now, we need to map it */
> + pte = pte_offset_map(pmd, _address);

No, it's within the same pte page table. It should be mapped with
pte_offset_map() above.

> + }
> + }
> + pte--;
> + pte_unmap(pte);
> + trace_mm_collapse_huge_page_swapin(mm, vma->vm_start, swap_pte);
> +}
> +
> static void collapse_huge_page(struct mm_struct *mm,
> unsigned long address,
> struct page **hpage,
> @@ -2551,6 +2586,8 @@ static void collapse_huge_page(struct mm_struct *mm,
> if (!pmd)
> goto out;
>
> + __collapse_huge_page_swapin(mm, vma, address, pmd, pte);
> +

And now the pages we swapped in are not isolated, right?
What prevents them from being swapped out again or whatever?

> anon_vma_lock_write(vma->anon_vma);
>
> pte = pte_offset_map(pmd, address);
> diff --git a/mm/memory.c b/mm/memory.c
> index e1c45d0..d801dc5 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2443,7 +2443,7 @@ 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)
> {
> --
> 1.9.1
>
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--
Kirill A. Shutemov