Re: [RFC V2] mm: change mm_advise_free to clear page dirty

[Michal Hocko]

On Sat 28-02-15 14:01:46, Wang, Yalin wrote:
> This patch add ClearPageDirty() to clear AnonPage dirty flag,
> if not clear page dirty for this anon page, the page will never be
> treated as freeable. we also make sure the shared AnonPage is not
> freeable, we implement it by dirty all copyed AnonPage pte,
> so that make sure the Anonpage will not become freeable, unless
> all process which shared this page call madvise_free syscall.

I am not able to parse this text.

> Another change is that we also handle file map page,
> we just clear pte young bit for file map, this is useful,
> it can make reclaim patch move file pages into inactive
> lru list aggressively.

This doesn't belong to this patch. If file private mappings should allow
MADV_FREE is a separate topic and should be discussed independently.

> 
> Signed-off-by: Yalin Wang <yalin.wang@xxxxxxxxxxxxxx>
> ---
>  mm/madvise.c | 26 +++++++++++++++-----------
>  mm/memory.c  | 12 ++++++++++--
>  2 files changed, 25 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 6d0fcb8..712756b 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -299,30 +299,38 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>  		page = vm_normal_page(vma, addr, ptent);
>  		if (!page)
>  			continue;
> +		if (!PageAnon(page))
> +			goto set_pte;
> +		if (!trylock_page(page))
> +			continue;
>  
>  		if (PageSwapCache(page)) {
> -			if (!trylock_page(page))
> -				continue;
> -
>  			if (!try_to_free_swap(page)) {
>  				unlock_page(page);
>  				continue;
>  			}
> -
> -			ClearPageDirty(page);
> -			unlock_page(page);
>  		}
>  
>  		/*
> +		 * we clear page dirty flag for AnonPage, no matter if this
> +		 * page is in swapcahce or not, AnonPage not in swapcache also set
> +		 * dirty flag sometimes, this happened when an AnonPage is removed
> +		 * from swapcahce by try_to_free_swap()
> +		 */
> +		ClearPageDirty(page);
> +		unlock_page(page);
> +		/*
>  		 * Some of architecture(ex, PPC) don't update TLB
>  		 * with set_pte_at and tlb_remove_tlb_entry so for
>  		 * the portability, remap the pte with old|clean
>  		 * after pte clearing.
>  		 */
> +set_pte:
>  		ptent = ptep_get_and_clear_full(mm, addr, pte,
>  						tlb->fullmm);
>  		ptent = pte_mkold(ptent);
> -		ptent = pte_mkclean(ptent);
> +		if (PageAnon(page))
> +			ptent = pte_mkclean(ptent);
>  		set_pte_at(mm, addr, pte, ptent);
>  		tlb_remove_tlb_entry(tlb, pte, addr);
>  	}
> @@ -364,10 +372,6 @@ static int madvise_free_single_vma(struct vm_area_struct *vma,
>  	if (vma->vm_flags & (VM_LOCKED|VM_HUGETLB|VM_PFNMAP))
>  		return -EINVAL;
>  
> -	/* MADV_FREE works for only anon vma at the moment */
> -	if (vma->vm_file)
> -		return -EINVAL;
> -
>  	start = max(vma->vm_start, start_addr);
>  	if (start >= vma->vm_end)
>  		return -EINVAL;
> diff --git a/mm/memory.c b/mm/memory.c
> index 8068893..3d949b3 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -874,10 +874,18 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>  	if (page) {
>  		get_page(page);
>  		page_dup_rmap(page);
> -		if (PageAnon(page))
> +		if (PageAnon(page)) {
> +			/*
> +			 * we dirty the copyed pte for anon page,
> +			 * this is useful for madvise_free_pte_range(),
> +			 * this can prevent shared anon page freed by madvise_free
> +			 * syscall
> +			 */
> +			pte = pte_mkdirty(pte);
>  			rss[MM_ANONPAGES]++;
> -		else
> +		} else {
>  			rss[MM_FILEPAGES]++;
> +		}
>  	}
>  
>  out_set_pte:
> -- 
> 2.2.2
> --
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-- 
Michal Hocko
SUSE Labs
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