[PATCH] [for mmotm-1113] mm: Simplify try_to_unmap_one()

[PATCH] [for mmotm-1113] mm: Simplify try_to_unmap_one()

[KOSAKI Motohiro]

SWAP_MLOCK mean "We marked the page as PG_MLOCK, please move it to
unevictable-lru". So, following code is easy confusable.

        if (vma->vm_flags & VM_LOCKED) {
                ret = SWAP_MLOCK;
                goto out_unmap;
        }

Plus, if the VMA doesn't have VM_LOCKED, We don't need to check
the needed of calling mlock_vma_page().

Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
 mm/rmap.c |   26 +++++++++++++-------------
 1 files changed, 13 insertions(+), 13 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 82e31fb..70dec01 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -779,10 +779,9 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 	 * skipped over this mm) then we should reactivate it.
 	 */
 	if (!(flags & TTU_IGNORE_MLOCK)) {
-		if (vma->vm_flags & VM_LOCKED) {
-			ret = SWAP_MLOCK;
-			goto out_unmap;
-		}
+		if (vma->vm_flags & VM_LOCKED)
+			goto out_mlock;
+
 		if (TTU_ACTION(flags) == TTU_MUNLOCK)
 			goto out_unmap;
 	}
@@ -855,18 +854,19 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 
 out_unmap:
 	pte_unmap_unlock(pte, ptl);
+out:
+	return ret;
 
-	if (ret == SWAP_MLOCK) {
-		ret = SWAP_AGAIN;
-		if (down_read_trylock(&vma->vm_mm->mmap_sem)) {
-			if (vma->vm_flags & VM_LOCKED) {
-				mlock_vma_page(page);
-				ret = SWAP_MLOCK;
-			}
-			up_read(&vma->vm_mm->mmap_sem);
+out_mlock:
+	pte_unmap_unlock(pte, ptl);
+
+	if (down_read_trylock(&vma->vm_mm->mmap_sem)) {
+		if (vma->vm_flags & VM_LOCKED) {
+			mlock_vma_page(page);
+			ret = SWAP_MLOCK;
 		}
+		up_read(&vma->vm_mm->mmap_sem);
 	}
-out:
 	return ret;
 }
 
-- 
1.6.2.5

Re: [PATCH] [for mmotm-1113] mm: Simplify try_to_unmap_one()

[Hugh Dickins]

On Tue, 17 Nov 2009, KOSAKI Motohiro wrote:

> SWAP_MLOCK mean "We marked the page as PG_MLOCK, please move it to
> unevictable-lru". So, following code is easy confusable.
> 
>         if (vma->vm_flags & VM_LOCKED) {
>                 ret = SWAP_MLOCK;
>                 goto out_unmap;
>         }
> 
> Plus, if the VMA doesn't have VM_LOCKED, We don't need to check
> the needed of calling mlock_vma_page().
> 
> Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

Acked-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>

> ---
>  mm/rmap.c |   26 +++++++++++++-------------
>  1 files changed, 13 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 82e31fb..70dec01 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -779,10 +779,9 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>  	 * skipped over this mm) then we should reactivate it.
>  	 */
>  	if (!(flags & TTU_IGNORE_MLOCK)) {
> -		if (vma->vm_flags & VM_LOCKED) {
> -			ret = SWAP_MLOCK;
> -			goto out_unmap;
> -		}
> +		if (vma->vm_flags & VM_LOCKED)
> +			goto out_mlock;
> +
>  		if (TTU_ACTION(flags) == TTU_MUNLOCK)
>  			goto out_unmap;
>  	}
> @@ -855,18 +854,19 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>  
>  out_unmap:
>  	pte_unmap_unlock(pte, ptl);
> +out:
> +	return ret;
>  
> -	if (ret == SWAP_MLOCK) {
> -		ret = SWAP_AGAIN;
> -		if (down_read_trylock(&vma->vm_mm->mmap_sem)) {
> -			if (vma->vm_flags & VM_LOCKED) {
> -				mlock_vma_page(page);
> -				ret = SWAP_MLOCK;
> -			}
> -			up_read(&vma->vm_mm->mmap_sem);
> +out_mlock:
> +	pte_unmap_unlock(pte, ptl);
> +
> +	if (down_read_trylock(&vma->vm_mm->mmap_sem)) {
> +		if (vma->vm_flags & VM_LOCKED) {
> +			mlock_vma_page(page);
> +			ret = SWAP_MLOCK;
>  		}
> +		up_read(&vma->vm_mm->mmap_sem);
>  	}
> -out:
>  	return ret;
>  }
>  
> -- 
> 1.6.2.5

Re: [PATCH] [for mmotm-1113] mm: Simplify try_to_unmap_one()

[Andrew Morton]

On Tue, 17 Nov 2009 17:39:27 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

> +out_mlock:
> +	pte_unmap_unlock(pte, ptl);
> +
> +	if (down_read_trylock(&vma->vm_mm->mmap_sem)) {
> +		if (vma->vm_flags & VM_LOCKED) {
> +			mlock_vma_page(page);
> +			ret = SWAP_MLOCK;
>  		}
> +		up_read(&vma->vm_mm->mmap_sem);

It's somewhat unobvious why we're using a trylock here.  Ranking versus
lock_page(), perhaps?

In general I think a trylock should have an associated comment which explains

a) why it is being used at this site and

b) what happens when the trylock fails - why this isn't a
   bug, how the kernel recovers from the inconsistency, what its
   overall effect is, etc.

<wonders why we need to take mmap_sem here at all>

Re: [PATCH] [for mmotm-1113] mm: Simplify try_to_unmap_one()

[KOSAKI Motohiro]

> On Tue, 17 Nov 2009 17:39:27 +0900 (JST)
> KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:
> 
> > +out_mlock:
> > +	pte_unmap_unlock(pte, ptl);
> > +
> > +	if (down_read_trylock(&vma->vm_mm->mmap_sem)) {
> > +		if (vma->vm_flags & VM_LOCKED) {
> > +			mlock_vma_page(page);
> > +			ret = SWAP_MLOCK;
> >  		}
> > +		up_read(&vma->vm_mm->mmap_sem);
> 
> It's somewhat unobvious why we're using a trylock here.  Ranking versus
> lock_page(), perhaps?
> 
> In general I think a trylock should have an associated comment which explains
> 
> a) why it is being used at this site and
> 
> b) what happens when the trylock fails - why this isn't a
>    bug, how the kernel recovers from the inconsistency, what its
>    overall effect is, etc.
> 
> <wonders why we need to take mmap_sem here at all>

This mmap_sem is needed certainenaly. Following comment is sufficient?


---
 mm/rmap.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 70dec01..b1c9342 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -860,6 +860,14 @@ out:
 out_mlock:
 	pte_unmap_unlock(pte, ptl);
 
+
+	/*
+	 * We need mmap_sem locking, Otherwise VM_LOCKED check makes
+	 * unstable result and race. Plus, We can't wait here because
+	 * we now hold anon_vma->lock or mapping->i_mmap_lock.
+	 * If trylock failed, The page remain evictable lru and
+	 * retry to more unevictable lru by later vmscan.
+	 */
 	if (down_read_trylock(&vma->vm_mm->mmap_sem)) {
 		if (vma->vm_flags & VM_LOCKED) {
 			mlock_vma_page(page);
-- 
1.6.2.5