Re: [PATCH 1/5] pagewalk: only split huge pages when necessary

[Mel Gorman <mel@csn.ul.ie>]

On Wed, Feb 09, 2011 at 11:54:07AM -0800, Dave Hansen wrote:
> 
> v2 - rework if() block, and remove  now redundant split_huge_page()
> 
> Right now, if a mm_walk has either ->pte_entry or ->pmd_entry
> set, it will unconditionally split any transparent huge pages
> it runs in to.  In practice, that means that anyone doing a
> 
> 	cat /proc/$pid/smaps
> 
> will unconditionally break down every huge page in the process
> and depend on khugepaged to re-collapse it later.  This is
> fairly suboptimal.
> 
> This patch changes that behavior.  It teaches each ->pmd_entry
> handler (there are five) that they must break down the THPs
> themselves.  Also, the _generic_ code will never break down
> a THP unless a ->pte_entry handler is actually set.
> 
> This means that the ->pmd_entry handlers can now choose to
> deal with THPs without breaking them down.
> 
> Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>
> ---
> 
>  linux-2.6.git-dave/fs/proc/task_mmu.c |    6 ++++++
>  linux-2.6.git-dave/include/linux/mm.h |    3 +++
>  linux-2.6.git-dave/mm/memcontrol.c    |    5 +++--
>  linux-2.6.git-dave/mm/pagewalk.c      |   24 ++++++++++++++++++++----
>  4 files changed, 32 insertions(+), 6 deletions(-)
> 
> diff -puN fs/proc/task_mmu.c~pagewalk-dont-always-split-thp fs/proc/task_mmu.c
> --- linux-2.6.git/fs/proc/task_mmu.c~pagewalk-dont-always-split-thp	2011-02-09 11:41:42.299558364 -0800
> +++ linux-2.6.git-dave/fs/proc/task_mmu.c	2011-02-09 11:41:42.319558349 -0800
> @@ -343,6 +343,8 @@ static int smaps_pte_range(pmd_t *pmd, u
>  	struct page *page;
>  	int mapcount;
>  
> +	split_huge_page_pmd(walk->mm, pmd);
> +
>  	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
>  	for (; addr != end; pte++, addr += PAGE_SIZE) {
>  		ptent = *pte;
> @@ -467,6 +469,8 @@ static int clear_refs_pte_range(pmd_t *p
>  	spinlock_t *ptl;
>  	struct page *page;
>  
> +	split_huge_page_pmd(walk->mm, pmd);
> +
>  	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
>  	for (; addr != end; pte++, addr += PAGE_SIZE) {
>  		ptent = *pte;
> @@ -623,6 +627,8 @@ static int pagemap_pte_range(pmd_t *pmd,
>  	pte_t *pte;
>  	int err = 0;
>  
> +	split_huge_page_pmd(walk->mm, pmd);
> +
>  	/* find the first VMA at or above 'addr' */
>  	vma = find_vma(walk->mm, addr);
>  	for (; addr != end; addr += PAGE_SIZE) {
> diff -puN include/linux/mm.h~pagewalk-dont-always-split-thp include/linux/mm.h
> --- linux-2.6.git/include/linux/mm.h~pagewalk-dont-always-split-thp	2011-02-09 11:41:42.303558361 -0800
> +++ linux-2.6.git-dave/include/linux/mm.h	2011-02-09 11:41:42.323558346 -0800
> @@ -899,6 +899,9 @@ unsigned long unmap_vmas(struct mmu_gath
>   * @pgd_entry: if set, called for each non-empty PGD (top-level) entry
>   * @pud_entry: if set, called for each non-empty PUD (2nd-level) entry
>   * @pmd_entry: if set, called for each non-empty PMD (3rd-level) entry
> + * 	       this handler is required to be able to handle
> + * 	       pmd_trans_huge() pmds.  They may simply choose to
> + * 	       split_huge_page() instead of handling it explicitly.
>   * @pte_entry: if set, called for each non-empty PTE (4th-level) entry
>   * @pte_hole: if set, called for each hole at all levels
>   * @hugetlb_entry: if set, called for each hugetlb entry
> diff -puN mm/memcontrol.c~pagewalk-dont-always-split-thp mm/memcontrol.c
> --- linux-2.6.git/mm/memcontrol.c~pagewalk-dont-always-split-thp	2011-02-09 11:41:42.311558355 -0800
> +++ linux-2.6.git-dave/mm/memcontrol.c	2011-02-09 11:41:42.327558343 -0800
> @@ -4737,7 +4737,8 @@ static int mem_cgroup_count_precharge_pt
>  	pte_t *pte;
>  	spinlock_t *ptl;
>  
> -	VM_BUG_ON(pmd_trans_huge(*pmd));
> +	split_huge_page_pmd(walk->mm, pmd);
> +
>  	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
>  	for (; addr != end; pte++, addr += PAGE_SIZE)
>  		if (is_target_pte_for_mc(vma, addr, *pte, NULL))
> @@ -4899,8 +4900,8 @@ static int mem_cgroup_move_charge_pte_ra
>  	pte_t *pte;
>  	spinlock_t *ptl;
>  
> +	split_huge_page_pmd(walk->mm, pmd);
>  retry:
> -	VM_BUG_ON(pmd_trans_huge(*pmd));
>  	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
>  	for (; addr != end; addr += PAGE_SIZE) {
>  		pte_t ptent = *(pte++);

Before we goto this retry, there is at a cond_resched(). Just to confirm,
we are depending on mmap_sem to prevent khugepaged promoting this back to
a hugepage, right? I don't see a problem with that but I want to be
sure.

> diff -puN mm/pagewalk.c~pagewalk-dont-always-split-thp mm/pagewalk.c
> --- linux-2.6.git/mm/pagewalk.c~pagewalk-dont-always-split-thp	2011-02-09 11:41:42.315558352 -0800
> +++ linux-2.6.git-dave/mm/pagewalk.c	2011-02-09 11:41:42.331558340 -0800
> @@ -33,19 +33,35 @@ static int walk_pmd_range(pud_t *pud, un
>  
>  	pmd = pmd_offset(pud, addr);
>  	do {
> +	again:
>  		next = pmd_addr_end(addr, end);
> -		split_huge_page_pmd(walk->mm, pmd);
> -		if (pmd_none_or_clear_bad(pmd)) {
> +		if (pmd_none(*pmd)) {
>  			if (walk->pte_hole)
>  				err = walk->pte_hole(addr, next, walk);
>  			if (err)
>  				break;
>  			continue;
>  		}
> +		/*
> +		 * This implies that each ->pmd_entry() handler
> +		 * needs to know about pmd_trans_huge() pmds
> +		 */
>  		if (walk->pmd_entry)
>  			err = walk->pmd_entry(pmd, addr, next, walk);
> -		if (!err && walk->pte_entry)
> -			err = walk_pte_range(pmd, addr, next, walk);
> +		if (err)
> +			break;
> +
> +		/*
> +		 * Check this here so we only break down trans_huge
> +		 * pages when we _need_ to
> +		 */
> +		if (!walk->pte_entry)
> +			continue;
> +
> +		split_huge_page_pmd(walk->mm, pmd);
> +		if (pmd_none_or_clear_bad(pmd))
> +			goto again;
> +		err = walk_pte_range(pmd, addr, next, walk);
>  		if (err)
>  			break;
>  	} while (pmd++, addr = next, addr != end);

-- 
Mel Gorman
SUSE Labs