[PATCH] fix return value for mb_cache_shrink_fn when nr_to_scan > 0

[PATCH] fix return value for mb_cache_shrink_fn when nr_to_scan > 0

[Wang Sheng-Hui]

Hi,

The comment for struct shrinker in include/linux/mm.h says
"shrink...It should return the number of objects which remain in the
cache."
Please notice the word "remain".

In fs/mbcache.h, mb_cache_shrink_fn is used as the shrink function:
 	static struct shrinker mb_cache_shrinker = {	
 		.shrink = mb_cache_shrink_fn,
 		.seeks = DEFAULT_SEEKS,
 	};
In mb_cache_shrink_fn, the return value for nr_to_scan > 0 is the
number of mb_cache_entry before shrink operation. It may because the
memory usage for mbcache is low, so the effect is not so obvious.
I think we'd better fix the return value issue.

Following patch is against 2.6.35-rc5. Please check it.

Signed-off-by: Wang Sheng-Hui <crosslonelyover@gmail.com>
---
 fs/mbcache.c |   10 ++++++++++
 1 files changed, 10 insertions(+), 0 deletions(-)

diff --git a/fs/mbcache.c b/fs/mbcache.c
index ec88ff3..412e7cc 100644
--- a/fs/mbcache.c
+++ b/fs/mbcache.c
@@ -228,6 +228,16 @@ mb_cache_shrink_fn(int nr_to_scan, gfp_t gfp_mask)
 		__mb_cache_entry_forget(list_entry(l, struct mb_cache_entry,
 						   e_lru_list), gfp_mask);
 	}
+	spin_lock(&mb_cache_spinlock);
+	count = 0;
+	list_for_each(l, &mb_cache_list) {
+		struct mb_cache *cache =
+			list_entry(l, struct mb_cache, c_cache_list);
+		mb_debug("cache %s (%d)", cache->c_name,
+			  atomic_read(&cache->c_entry_count));
+		count += atomic_read(&cache->c_entry_count);
+	}
+	spin_unlock(&mb_cache_spinlock);
 out:
 	return (count / 100) * sysctl_vfs_cache_pressure;
 }
-- 
1.7.1.1





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Re: [PATCH] fix return value for mb_cache_shrink_fn when nr_to_scan > 0

[Eric Sandeen]

Wang Sheng-Hui wrote:

> Hi,
>
> The comment for struct shrinker in include/linux/mm.h says
> "shrink...It should return the number of objects which remain in the
> cache."
> Please notice the word "remain".
>
> In fs/mbcache.h, mb_cache_shrink_fn is used as the shrink function:
>  	static struct shrinker mb_cache_shrinker = {	
>  		.shrink = mb_cache_shrink_fn,
>  		.seeks = DEFAULT_SEEKS,
>  	};
> In mb_cache_shrink_fn, the return value for nr_to_scan > 0 is the
> number of mb_cache_entry before shrink operation. It may because the
> memory usage for mbcache is low, so the effect is not so obvious.
> I think we'd better fix the return value issue.
>
> Following patch is against 2.6.35-rc5. Please check it.
>
>   
you are right that it's not returning the remaining entries, but I think
we can do this more simply; there isn't any reason to calculate it twice
How about just moving the accounting to the end, since "count" isn't actually
used when freeing, anyway.... something like this?

diff --git a/fs/mbcache.c b/fs/mbcache.c
index ec88ff3..3af79de 100644
--- a/fs/mbcache.c
+++ b/fs/mbcache.c
@@ -203,19 +203,11 @@ mb_cache_shrink_fn(int nr_to_scan, gfp_t gfp_mask)
 	struct list_head *l, *ltmp;
 	int count = 0;
 
-	spin_lock(&mb_cache_spinlock);
-	list_for_each(l, &mb_cache_list) {
-		struct mb_cache *cache =
-			list_entry(l, struct mb_cache, c_cache_list);
-		mb_debug("cache %s (%d)", cache->c_name,
-			  atomic_read(&cache->c_entry_count));
-		count += atomic_read(&cache->c_entry_count);
-	}
 	mb_debug("trying to free %d entries", nr_to_scan);
-	if (nr_to_scan == 0) {
-		spin_unlock(&mb_cache_spinlock);
+	if (nr_to_scan == 0)
 		goto out;
-	}
+
+	spin_lock &mb_cache_spinlock);
 	while (nr_to_scan-- && !list_empty(&mb_cache_lru_list)) {
 		struct mb_cache_entry *ce =
 			list_entry(mb_cache_lru_list.next,
@@ -229,6 +221,17 @@ mb_cache_shrink_fn(int nr_to_scan, gfp_t gfp_mask)
 						   e_lru_list), gfp_mask);
 	}
 out:
+	/* Count remaining entries */
+	spin_lock(&mb_cache_spinlock);
+	list_for_each(l, &mb_cache_list) {
+		struct mb_cache *cache =
+			list_entry(l, struct mb_cache, c_cache_list);
+		mb_debug("cache %s (%d)", cache->c_name,
+			  atomic_read(&cache->c_entry_count));
+		count += atomic_read(&cache->c_entry_count);
+	}
+	spin_unlock(&mb_cache_spinlock);
+
 	return (count / 100) * sysctl_vfs_cache_pressure;
 }
 

Re: [PATCH] fix return value for mb_cache_shrink_fn when nr_to_scan > 0

[Christoph Hellwig]

On Sat, Jul 17, 2010 at 11:06:32PM -0500, Eric Sandeen wrote:
> +	/* Count remaining entries */
> +	spin_lock(&mb_cache_spinlock);
> +	list_for_each(l, &mb_cache_list) {
> +		struct mb_cache *cache =
> +			list_entry(l, struct mb_cache, c_cache_list);

This should be using list_for_each_entry.

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Re: [PATCH] fix return value for mb_cache_shrink_fn when nr_to_scan > 0

[Wang Sheng-Hui]

于 2010-7-18 14:01, Christoph Hellwig 写道:
> On Sat, Jul 17, 2010 at 11:06:32PM -0500, Eric Sandeen wrote:
>> +	/* Count remaining entries */
>> +	spin_lock(&mb_cache_spinlock);
>> +	list_for_each(l,&mb_cache_list) {
>> +		struct mb_cache *cache =
>> +			list_entry(l, struct mb_cache, c_cache_list);
>
> This should be using list_for_each_entry.
>

I regenerated the patch. Please check it.

Signed-off-by: Wang Sheng-Hui <crosslonelyover@gmail.com>
---
  fs/mbcache.c |   22 +++++++++++-----------
  1 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/fs/mbcache.c b/fs/mbcache.c
index ec88ff3..5697d9e 100644
--- a/fs/mbcache.c
+++ b/fs/mbcache.c
@@ -201,21 +201,13 @@ mb_cache_shrink_fn(int nr_to_scan, gfp_t gfp_mask)
  {
  	LIST_HEAD(free_list);
  	struct list_head *l, *ltmp;
+	struct mb_cache *cache;
  	int count = 0;

-	spin_lock(&mb_cache_spinlock);
-	list_for_each(l, &mb_cache_list) {
-		struct mb_cache *cache =
-			list_entry(l, struct mb_cache, c_cache_list);
-		mb_debug("cache %s (%d)", cache->c_name,
-			  atomic_read(&cache->c_entry_count));
-		count += atomic_read(&cache->c_entry_count);
-	}
  	mb_debug("trying to free %d entries", nr_to_scan);
-	if (nr_to_scan == 0) {
-		spin_unlock(&mb_cache_spinlock);
+	if (nr_to_scan == 0)
  		goto out;
-	}
+
  	while (nr_to_scan-- && !list_empty(&mb_cache_lru_list)) {
  		struct mb_cache_entry *ce =
  			list_entry(mb_cache_lru_list.next,
@@ -229,6 +221,14 @@ mb_cache_shrink_fn(int nr_to_scan, gfp_t gfp_mask)
  						   e_lru_list), gfp_mask);
  	}
  out:
+	spin_lock(&mb_cache_spinlock);
+	list_for_each_entry(cache, &mb_cache_list, c_cache_list) {
+		mb_debug("cache %s (%d)", cache->c_name,
+			  atomic_read(&cache->c_entry_count));
+		count += atomic_read(&cache->c_entry_count);
+	}
+	spin_unlock(&mb_cache_spinlock);
+
  	return (count / 100) * sysctl_vfs_cache_pressure;
  }

-- 
1.7.1.1



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Re: [PATCH] fix return value for mb_cache_shrink_fn when nr_to_scan > 0

[Andreas Gruenbacher]

On Sunday 18 July 2010 08:36:59 Wang Sheng-Hui wrote:
> I regenerated the patch. Please check it.

The logic for calculating how many objects to free is still wrong: 
mb_cache_shrink_fn returns the number of entries scaled by 
sysctl_vfs_cache_pressure / 100.  It should also scale nr_to_scan by the 
inverse of that.  The sysctl_vfs_cache_pressure == 0 case (never scale) may 
require special attention.

See dcache_shrinker() in fs/dcache.c.

Thanks,
Andreas

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Re: [PATCH] fix return value for mb_cache_shrink_fn when nr_to_scan > 0

[Andreas Gruenbacher]

On Sunday 18 July 2010 08:36:59 Wang Sheng-Hui wrote:
> 于 2010-7-18 14:01, Christoph Hellwig 写道:
> > This should be using list_for_each_entry.

It would make sense to change this throughout the whole file.

Thanks,
Andreas

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Re: [PATCH] fix return value for mb_cache_shrink_fn when nr_to_scan > 0

[shenghui]

2010/7/20 Andreas Gruenbacher <agruen@suse.de>:
> On Sunday 18 July 2010 08:36:59 Wang Sheng-Hui wrote:
>> I regenerated the patch. Please check it.
>
> The logic for calculating how many objects to free is still wrong:
> mb_cache_shrink_fn returns the number of entries scaled by
> sysctl_vfs_cache_pressure / 100.  It should also scale nr_to_scan by the
> inverse of that.  The sysctl_vfs_cache_pressure == 0 case (never scale) may
> require special attention.
>
> See dcache_shrinker() in fs/dcache.c.
>
> Thanks,
> Andreas
>

Sorry, I haven't found any special attention on
sysctl_vfs_cache_pressure == 0 case or scale
nr_to_scan in fs/dcache.c

 900static int shrink_dcache_memory(int nr, gfp_t gfp_mask)
 901{
 902        if (nr) {
 903                if (!(gfp_mask & __GFP_FS))
 904                        return -1;
 905                prune_dcache(nr);
 906        }
 907        return (dentry_stat.nr_unused / 100) * sysctl_vfs_cache_pressure;
 908}




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Re: [PATCH] fix return value for mb_cache_shrink_fn when nr_to_scan > 0

[shenghui]

2010/7/20 shenghui <crosslonelyover@gmail.com>:
> 2010/7/20 Andreas Gruenbacher <agruen@suse.de>:
>
> Sorry, I haven't found any special attention on
> sysctl_vfs_cache_pressure == 0 case or scale
> nr_to_scan in fs/dcache.c
>
>  900static int shrink_dcache_memory(int nr, gfp_t gfp_mask)
>  901{
>  902        if (nr) {
>  903                if (!(gfp_mask & __GFP_FS))
>  904                        return -1;
>  905                prune_dcache(nr);
>  906        }
>  907        return (dentry_stat.nr_unused / 100) * sysctl_vfs_cache_pressure;
>  908}
>

And for sysctl_vfs_cache_pressure == 0 case, it's
enough to return 0 to indicate no cache entries left.



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Re: [PATCH] fix return value for mb_cache_shrink_fn when nr_to_scan > 0

[Eric Sandeen]

Andreas Gruenbacher wrote:
> On Sunday 18 July 2010 08:36:59 Wang Sheng-Hui wrote:
>> I regenerated the patch. Please check it.
> 
> The logic for calculating how many objects to free is still wrong: 
> mb_cache_shrink_fn returns the number of entries scaled by 
> sysctl_vfs_cache_pressure / 100.  It should also scale nr_to_scan by the 
> inverse of that.  The sysctl_vfs_cache_pressure == 0 case (never scale) may 
> require special attention.

I don't think that's right:

vfs_cache_pressure
------------------

Controls the tendency of the kernel to reclaim the memory which is used for
caching of directory and inode objects.

At the default value of vfs_cache_pressure=100 the kernel will attempt to
reclaim dentries and inodes at a "fair" rate with respect to pagecache and
swapcache reclaim.  Decreasing vfs_cache_pressure causes the kernel to prefer
to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will
never reclaim dentries and inodes due to memory pressure and this can easily
lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100
causes the kernel to prefer to reclaim dentries and inodes.


0 means "never reclaim," it doesn't mean "never scale."

As for nr_to_scan, after the first call, the shrinker has a scaled
version of the total count, so the requested nr_to_scan on the
next call is already scaled based on that.

I think the logic in the mbcache shrinker is fine.

-Eric

> See dcache_shrinker() in fs/dcache.c.



> 
> Thanks,
> Andreas

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Re: [PATCH] fix return value for mb_cache_shrink_fn when nr_to_scan > 0

[Andreas Gruenbacher]

On Tuesday 20 July 2010 17:13:56 Eric Sandeen wrote:
> I think the logic in the mbcache shrinker is fine.

Indeed yes, I got confused, sorry.

On Sunday 18 July 2010 08:36:59 Wang Sheng-Hui wrote:
> I regenerated the patch. Please check it.

Sheng-Hui, the mb_cache_lru_list list is now accessed without holding 
mb_cache_spinlock.

Thanks,
Andreas

[PATCH] fix return value for mb_cache_shrink_fn when nr_to_scan > 0

[Wang Sheng-Hui]

Sorry. regerated the patch, please check it.
I wrapped most code in single pair of spinlock ops for 2 reasons:
1) get spinlock 2 times seems time consuming
2) use single pair of spinlock ops can keep "count"
   consistent for the shrink operation. 2 pairs may
   get some new ces created by other processes. 



Signed-off-by: Wang Sheng-Hui <crosslonelyover@gmail.com>
---
 fs/mbcache.c |   24 ++++++++++++------------
 1 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/fs/mbcache.c b/fs/mbcache.c
index ec88ff3..ee57aa3 100644
--- a/fs/mbcache.c
+++ b/fs/mbcache.c
@@ -201,21 +201,15 @@ mb_cache_shrink_fn(int nr_to_scan, gfp_t gfp_mask)
 {
 	LIST_HEAD(free_list);
 	struct list_head *l, *ltmp;
+	struct mb_cache *cache;
 	int count = 0;
 
-	spin_lock(&mb_cache_spinlock);
-	list_for_each(l, &mb_cache_list) {
-		struct mb_cache *cache =
-			list_entry(l, struct mb_cache, c_cache_list);
-		mb_debug("cache %s (%d)", cache->c_name,
-			  atomic_read(&cache->c_entry_count));
-		count += atomic_read(&cache->c_entry_count);
-	}
 	mb_debug("trying to free %d entries", nr_to_scan);
-	if (nr_to_scan == 0) {
-		spin_unlock(&mb_cache_spinlock);
+
+	spin_lock(&mb_cache_spinlock);
+	if (nr_to_scan == 0)
 		goto out;
-	}
+
 	while (nr_to_scan-- && !list_empty(&mb_cache_lru_list)) {
 		struct mb_cache_entry *ce =
 			list_entry(mb_cache_lru_list.next,
@@ -223,12 +217,18 @@ mb_cache_shrink_fn(int nr_to_scan, gfp_t gfp_mask)
 		list_move_tail(&ce->e_lru_list, &free_list);
 		__mb_cache_entry_unhash(ce);
 	}
-	spin_unlock(&mb_cache_spinlock);
 	list_for_each_safe(l, ltmp, &free_list) {
 		__mb_cache_entry_forget(list_entry(l, struct mb_cache_entry,
 						   e_lru_list), gfp_mask);
 	}
 out:
+	list_for_each_entry(cache, &mb_cache_list, c_cache_list) {
+		mb_debug("cache %s (%d)", cache->c_name,
+			  atomic_read(&cache->c_entry_count));
+		count += atomic_read(&cache->c_entry_count);
+	}
+	spin_unlock(&mb_cache_spinlock);
+
 	return (count / 100) * sysctl_vfs_cache_pressure;
 }
 
-- 
1.6.3.3

Re: [PATCH] fix return value for mb_cache_shrink_fn when nr_to_scan > 0

[Eric Sandeen]

Wang Sheng-Hui wrote:
> Sorry. regerated the patch, please check it.
> I wrapped most code in single pair of spinlock ops for 2 reasons:
> 1) get spinlock 2 times seems time consuming
> 2) use single pair of spinlock ops can keep "count"
>   consistent for the shrink operation. 2 pairs may
>   get some new ces created by other processes.
> 

Sorry, this patch appears to have whitespace cut & paste mangling.

More comments below.

> Signed-off-by: Wang Sheng-Hui <crosslonelyover@gmail.com>
> ---
> fs/mbcache.c |   24 ++++++++++++------------
> 1 files changed, 12 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/mbcache.c b/fs/mbcache.c
> index ec88ff3..ee57aa3 100644
> --- a/fs/mbcache.c
> +++ b/fs/mbcache.c
> @@ -201,21 +201,15 @@ mb_cache_shrink_fn(int nr_to_scan, gfp_t gfp_mask)
> {
>     LIST_HEAD(free_list);
>     struct list_head *l, *ltmp;
> +    struct mb_cache *cache;
>     int count = 0;
> 
> -    spin_lock(&mb_cache_spinlock);
> -    list_for_each(l, &mb_cache_list) {
> -        struct mb_cache *cache =
> -            list_entry(l, struct mb_cache, c_cache_list);
> -        mb_debug("cache %s (%d)", cache->c_name,
> -              atomic_read(&cache->c_entry_count));
> -        count += atomic_read(&cache->c_entry_count);
> -    }
>     mb_debug("trying to free %d entries", nr_to_scan);
> -    if (nr_to_scan == 0) {
> -        spin_unlock(&mb_cache_spinlock);
> +
> +    spin_lock(&mb_cache_spinlock);
> +    if (nr_to_scan == 0)
>         goto out;
> -    }
> +
>     while (nr_to_scan-- && !list_empty(&mb_cache_lru_list)) {
>         struct mb_cache_entry *ce =
>             list_entry(mb_cache_lru_list.next,
> @@ -223,12 +217,18 @@ mb_cache_shrink_fn(int nr_to_scan, gfp_t gfp_mask)
>         list_move_tail(&ce->e_lru_list, &free_list);
>         __mb_cache_entry_unhash(ce);
>     }
> -    spin_unlock(&mb_cache_spinlock);

you can't do this because

>     list_for_each_safe(l, ltmp, &free_list) {
>         __mb_cache_entry_forget(list_entry(l, struct mb_cache_entry,

this takes the spinlock too and you'll deadlock.

Did you test this patch?

-Eric

>                            e_lru_list), gfp_mask);
>     }
> out:
> +    list_for_each_entry(cache, &mb_cache_list, c_cache_list) {
> +        mb_debug("cache %s (%d)", cache->c_name,
> +              atomic_read(&cache->c_entry_count));
> +        count += atomic_read(&cache->c_entry_count);
> +    }
> +    spin_unlock(&mb_cache_spinlock);
> +
>     return (count / 100) * sysctl_vfs_cache_pressure;
> }
> 

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re: [PATCH] fix return value for mb_cache_shrink_fn when nr_to_scan > 0

[Wang Sheng-Hui]

Sorry, missed that. Regerated and passed checkpatch.pl check. 
Please check it.


Signed-off-by: Wang Sheng-Hui <crosslonelyover@gmail.com>
---
 fs/mbcache.c |   23 ++++++++++++-----------
 1 files changed, 12 insertions(+), 11 deletions(-)

diff --git a/fs/mbcache.c b/fs/mbcache.c
index ec88ff3..603170e 100644
--- a/fs/mbcache.c
+++ b/fs/mbcache.c
@@ -201,21 +201,14 @@ mb_cache_shrink_fn(int nr_to_scan, gfp_t gfp_mask)
 {
 	LIST_HEAD(free_list);
 	struct list_head *l, *ltmp;
+	struct mb_cache *cache;
 	int count = 0;
 
-	spin_lock(&mb_cache_spinlock);
-	list_for_each(l, &mb_cache_list) {
-		struct mb_cache *cache =
-			list_entry(l, struct mb_cache, c_cache_list);
-		mb_debug("cache %s (%d)", cache->c_name,
-			  atomic_read(&cache->c_entry_count));
-		count += atomic_read(&cache->c_entry_count);
-	}
 	mb_debug("trying to free %d entries", nr_to_scan);
-	if (nr_to_scan == 0) {
-		spin_unlock(&mb_cache_spinlock);
+	if (nr_to_scan == 0)
 		goto out;
-	}
+
+	spin_lock(&mb_cache_spinlock);
 	while (nr_to_scan-- && !list_empty(&mb_cache_lru_list)) {
 		struct mb_cache_entry *ce =
 			list_entry(mb_cache_lru_list.next,
@@ -229,6 +222,14 @@ mb_cache_shrink_fn(int nr_to_scan, gfp_t gfp_mask)
 						   e_lru_list), gfp_mask);
 	}
 out:
+	spin_lock(&mb_cache_spinlock);
+	list_for_each_entry(cache, &mb_cache_list, c_cache_list) {
+		mb_debug("cache %s (%d)", cache->c_name,
+			  atomic_read(&cache->c_entry_count));
+		count += atomic_read(&cache->c_entry_count);
+	}
+	spin_unlock(&mb_cache_spinlock);
+
 	return (count / 100) * sysctl_vfs_cache_pressure;
 }
 
-- 
1.6.3.3

Re: [PATCH] fix return value for mb_cache_shrink_fn when nr_to_scan > 0

[shenghui]

Sorry to trouble you all & Thanks for your instructions!
I noticed that Andreas Gruenbacher has submitted patches
on mbcache.
Please ignore mine.


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shenghui

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