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Date:	Fri, 24 Jun 2011 09:57:42 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Christoph Hellwig <hch@...radead.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Michel Lespinasse <walken@...gle.com>,
	Mel Gorman <mgorman@...e.de>, Lutz Vieweg <lvml@....de>
Subject: Re: [PATCH] mm: preallocate page before lock_page at filemap COW.
 (WasRe: [PATCH V2] mm: Do not keep page locked during page fault while
 charging it for memcg

On Thu 23-06-11 19:01:57, KAMEZAWA Hiroyuki wrote:
> From 7b0aa038f3a1c6a479a3ce6acb38c7d2740d7a75 Mon Sep 17 00:00:00 2001
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Date: Thu, 23 Jun 2011 18:50:32 +0900
> Subject: [PATCH] mm: preallocate page before lock_page() at filemap COW.
> 
> Currently we are keeping faulted page locked throughout whole __do_fault
> call (except for page_mkwrite code path) after calling file system's
> fault code. If we do early COW, we allocate a new page which has to be
> charged for a memcg (mem_cgroup_newpage_charge).
> 
> This function, however, might block for unbounded amount of time if memcg
> oom killer is disabled or fork-bomb is running because the only way out of
> the OOM situation is either an external event or OOM-situation fix.
> 
> In the end we are keeping the faulted page locked and blocking other
> processes from faulting it in which is not good at all because we are
> basically punishing potentially an unrelated process for OOM condition
> in a different group (I have seen stuck system because of ld-2.11.1.so being
> locked).
> 
> We can do test easily.
> 
>  % cgcreate -g memory:A
>  % cgset -r memory.limit_in_bytes=64M A
>  % cgset -r memory.memsw.limit_in_bytes=64M A
>  % cd kernel_dir; cgexec -g memory:A make -j
> 
> Then, the whole system will live-locked until you kill 'make -j'
> by hands (or push reboot...) This is because some important
> page in a shared library are locked.
> 
> Considering again, the new page is not necessary to be allocated
> with lock_page() held. So....
> 
> This patch moves "charge" and memory allocation for COW page
> before lock_page(). Then, we can avoid scanning LRU with holding
> a lock on a page.
> 
> Then, above livelock disappears.
> 
> Reported-by: Lutz Vieweg <lvml@....de>
> Original-idea-by: Michal Hocko <mhocko@...e.cz>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

Sorry, forgot to send my
Reviewed-by: Michal Hocko <mhocko@...e>

I still have concerns about this way to handle the issue. See the follow
up discussion in other thread (https://lkml.org/lkml/2011/6/23/135).

Anyway I think that we do not have many other options to handle this.
Either we unlock, charge, lock&restes or we preallocate, fault in

Or am I missing some other ways how to do it? What do others think about
these approaches?

> 
> Changelog:
>   - change the logic from "do charge after unlock" to
>     "do charge before lock".
> ---
>  mm/memory.c |   56 ++++++++++++++++++++++++++++++++++----------------------
>  1 files changed, 34 insertions(+), 22 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 87d9353..845378c 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3127,14 +3127,34 @@ static int __do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>  	pte_t *page_table;
>  	spinlock_t *ptl;
>  	struct page *page;
> +	struct page *cow_page;
>  	pte_t entry;
>  	int anon = 0;
> -	int charged = 0;
>  	struct page *dirty_page = NULL;
>  	struct vm_fault vmf;
>  	int ret;
>  	int page_mkwrite = 0;
>  
> +	/*
> +	 * If we do COW later, allocate page befor taking lock_page()
> +	 * on the file cache page. This will reduce lock holding time.
> +	 */
> +	if ((flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED)) {
> +
> +		if (unlikely(anon_vma_prepare(vma)))
> +			return  VM_FAULT_OOM;
> +
> +		cow_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);
> +		if (!cow_page)
> +			return VM_FAULT_OOM;
> +
> +		if (mem_cgroup_newpage_charge(cow_page, mm, GFP_KERNEL)) {
> +			page_cache_release(cow_page);
> +			return VM_FAULT_OOM;
> +		}
> +	} else
> +		cow_page = NULL;
> +
>  	vmf.virtual_address = (void __user *)(address & PAGE_MASK);
>  	vmf.pgoff = pgoff;
>  	vmf.flags = flags;
> @@ -3143,12 +3163,13 @@ static int __do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>  	ret = vma->vm_ops->fault(vma, &vmf);
>  	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE |
>  			    VM_FAULT_RETRY)))
> -		return ret;
> +		goto uncharge_out;
>  
>  	if (unlikely(PageHWPoison(vmf.page))) {
>  		if (ret & VM_FAULT_LOCKED)
>  			unlock_page(vmf.page);
> -		return VM_FAULT_HWPOISON;
> +		ret = VM_FAULT_HWPOISON;
> +		goto uncharge_out;
>  	}
>  
>  	/*
> @@ -3166,23 +3187,8 @@ static int __do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>  	page = vmf.page;
>  	if (flags & FAULT_FLAG_WRITE) {
>  		if (!(vma->vm_flags & VM_SHARED)) {
> +			page = cow_page;
>  			anon = 1;
> -			if (unlikely(anon_vma_prepare(vma))) {
> -				ret = VM_FAULT_OOM;
> -				goto out;
> -			}
> -			page = alloc_page_vma(GFP_HIGHUSER_MOVABLE,
> -						vma, address);
> -			if (!page) {
> -				ret = VM_FAULT_OOM;
> -				goto out;
> -			}
> -			if (mem_cgroup_newpage_charge(page, mm, GFP_KERNEL)) {
> -				ret = VM_FAULT_OOM;
> -				page_cache_release(page);
> -				goto out;
> -			}
> -			charged = 1;
>  			copy_user_highpage(page, vmf.page, address, vma);
>  			__SetPageUptodate(page);
>  		} else {
> @@ -3251,8 +3257,8 @@ static int __do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>  		/* no need to invalidate: a not-present page won't be cached */
>  		update_mmu_cache(vma, address, page_table);
>  	} else {
> -		if (charged)
> -			mem_cgroup_uncharge_page(page);
> +		if (cow_page)
> +			mem_cgroup_uncharge_page(cow_page);
>  		if (anon)
>  			page_cache_release(page);
>  		else
> @@ -3261,7 +3267,6 @@ static int __do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>  
>  	pte_unmap_unlock(page_table, ptl);
>  
> -out:
>  	if (dirty_page) {
>  		struct address_space *mapping = page->mapping;
>  
> @@ -3291,6 +3296,13 @@ out:
>  unwritable_page:
>  	page_cache_release(page);
>  	return ret;
> +uncharge_out:
> +	/* fs's fault handler get error */
> +	if (cow_page) {
> +		mem_cgroup_uncharge_page(cow_page);
> +		page_cache_release(cow_page);
> +	}
> +	return ret;
>  }
>  
>  static int do_linear_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> -- 
> 1.7.4.1
> 
> 
> 
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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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