linux-kernel - Re: [PATCH 2/2] uprobe: collapse THP pmd after removing all uprobes

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Date:   Tue, 30 Jul 2019 18:01:10 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Song Liu <songliubraving@...com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, matthew.wilcox@...cle.com,
        kirill.shutemov@...ux.intel.com, oleg@...hat.com,
        kernel-team@...com, william.kucharski@...cle.com,
        srikar@...ux.vnet.ibm.com
Subject: Re: [PATCH 2/2] uprobe: collapse THP pmd after removing all uprobes

On Sun, Jul 28, 2019 at 10:43:35PM -0700, Song Liu wrote:
> After all uprobes are removed from the huge page (with PTE pgtable), it
> is possible to collapse the pmd and benefit from THP again. This patch
> does the collapse by calling khugepaged_add_pte_mapped_thp().
> 
> Signed-off-by: Song Liu <songliubraving@...com>
> ---
>  kernel/events/uprobes.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 58ab7fc7272a..cc53789fefc6 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -26,6 +26,7 @@
>  #include <linux/percpu-rwsem.h>
>  #include <linux/task_work.h>
>  #include <linux/shmem_fs.h>
> +#include <linux/khugepaged.h>
>  
>  #include <linux/uprobes.h>
>  
> @@ -470,6 +471,7 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
>  	struct page *old_page, *new_page;
>  	struct vm_area_struct *vma;
>  	int ret, is_register, ref_ctr_updated = 0;
> +	bool orig_page_huge = false;
>  
>  	is_register = is_swbp_insn(&opcode);
>  	uprobe = container_of(auprobe, struct uprobe, arch);
> @@ -525,6 +527,9 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
>  
>  				/* dec_mm_counter for old_page */
>  				dec_mm_counter(mm, MM_ANONPAGES);
> +
> +				if (PageCompound(orig_page))
> +					orig_page_huge = true;
>  			}
>  			put_page(orig_page);
>  		}
> @@ -543,6 +548,10 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
>  	if (ret && is_register && ref_ctr_updated)
>  		update_ref_ctr(uprobe, mm, -1);
>  
> +	/* try collapse pmd for compound page */
> +	if (!ret && orig_page_huge)
> +		khugepaged_add_pte_mapped_thp(mm, vaddr & HPAGE_PMD_MASK);
> +

IIUC, here you have all locks taken, so you should be able to call
collapse_pte_mapped_thp() directly, shouldn't you?

-- 
 Kirill A. Shutemov