[PATCH v2 0/2] memcg: migration clean up

[PATCH v2 0/2] memcg: migration clean up

[Minchan Kim]

This patch cleans up memcg migration.
This patch sent Tue, Jan 11, 2011 but maybe Andrew was very busy so lost.
I resend with Acked-by and rebased on mmotm-2011-02-04.

Minchan Kim (2):
  memcg: remove unnecessary BUG_ON
  memcg: remove charge variable in unmap_and_move

 mm/memcontrol.c |    1 +
 mm/migrate.c    |   10 +++-------
 2 files changed, 4 insertions(+), 7 deletions(-)

[PATCH v2 0/2] memcg: migration clean up

[Minchan Kim]

This patch cleans up memcg migration.
This patch sent Tue, Jan 11, 2011 but maybe Andrew was very busy so lost.
I resend with Acked-by and rebased on mmotm-2011-02-04.

Minchan Kim (2):
  memcg: remove unnecessary BUG_ON
  memcg: remove charge variable in unmap_and_move

 mm/memcontrol.c |    1 +
 mm/migrate.c    |   10 +++-------
 2 files changed, 4 insertions(+), 7 deletions(-)

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[PATCH v2 1/2] memcg: remove unnecessary BUG_ON

[Minchan Kim]

Now memcg in unmap_and_move checks BUG_ON of charge.
But mem_cgroup_prepare_migration returns either 0 or -ENOMEM.
If it returns -ENOMEM, it jumps out unlock without the check.
If it returns 0, it can pass BUG_ON. So it's meaningless.
Let's remove it.

Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
---
 mm/migrate.c |    1 -
 1 files changed, 0 insertions(+), 1 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index eb083a6..2abc9c9 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -683,7 +683,6 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 		rc = -ENOMEM;
 		goto unlock;
 	}
-	BUG_ON(charge);
 
 	if (PageWriteback(page)) {
 		if (!force || !sync)
-- 
1.7.1

[PATCH v2 1/2] memcg: remove unnecessary BUG_ON

[Minchan Kim]

Now memcg in unmap_and_move checks BUG_ON of charge.
But mem_cgroup_prepare_migration returns either 0 or -ENOMEM.
If it returns -ENOMEM, it jumps out unlock without the check.
If it returns 0, it can pass BUG_ON. So it's meaningless.
Let's remove it.

Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
---
 mm/migrate.c |    1 -
 1 files changed, 0 insertions(+), 1 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index eb083a6..2abc9c9 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -683,7 +683,6 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 		rc = -ENOMEM;
 		goto unlock;
 	}
-	BUG_ON(charge);
 
 	if (PageWriteback(page)) {
 		if (!force || !sync)
-- 
1.7.1

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[PATCH v2 2/2] memcg: remove charge variable in unmap_and_move

[Minchan Kim]

memcg charge/uncharge could be handled by mem_cgroup_[prepare/end]
migration itself so charge local variable in unmap_and_move lost the role
since we introduced 01b1ae63c2.

In addition, the variable name is not good like below.

int unmap_and_move()
{
	charge = mem_cgroup_prepare_migration(xxx);
	..
		BUG_ON(charge); <-- BUG if it is charged?
		..
uncharge:
		if (!charge)    <-- why do we have to uncharge !charge?
			mem_group_end_migration(xxx);
	..
}

So let's remove unnecessary and confusing variable.

Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Suggested-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
---
 mm/memcontrol.c |    1 +
 mm/migrate.c    |    9 +++------
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8a97571..3c91d5c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2873,6 +2873,7 @@ static inline int mem_cgroup_move_swap_account(swp_entry_t entry,
 /*
  * Before starting migration, account PAGE_SIZE to mem_cgroup that the old
  * page belongs to.
+ * Note: Should not return -EAGAIN. unmap_and_move depens on it.
  */
 int mem_cgroup_prepare_migration(struct page *page,
 	struct page *newpage, struct mem_cgroup **ptr, gfp_t gfp_mask)
diff --git a/mm/migrate.c b/mm/migrate.c
index 2abc9c9..37055d0 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -622,7 +622,6 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 	int *result = NULL;
 	struct page *newpage = get_new_page(page, private, &result);
 	int remap_swapcache = 1;
-	int charge = 0;
 	struct mem_cgroup *mem;
 	struct anon_vma *anon_vma = NULL;
 
@@ -637,7 +636,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 		if (unlikely(split_huge_page(page)))
 			goto move_newpage;
 
-	/* prepare cgroup just returns 0 or -ENOMEM */
+	/* mem_cgroup_prepage_migration never returns -EAGAIN */
 	rc = -EAGAIN;
 
 	if (!trylock_page(page)) {
@@ -678,8 +677,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 	}
 
 	/* charge against new page */
-	charge = mem_cgroup_prepare_migration(page, newpage, &mem, GFP_KERNEL);
-	if (charge == -ENOMEM) {
+	if (mem_cgroup_prepare_migration(page, newpage, &mem, GFP_KERNEL)) {
 		rc = -ENOMEM;
 		goto unlock;
 	}
@@ -766,8 +764,7 @@ skip_unmap:
 		drop_anon_vma(anon_vma);
 
 uncharge:
-	if (!charge)
-		mem_cgroup_end_migration(mem, page, newpage, rc == 0);
+	mem_cgroup_end_migration(mem, page, newpage, rc == 0);
 unlock:
 	unlock_page(page);
 
-- 
1.7.1

[PATCH v2 2/2] memcg: remove charge variable in unmap_and_move

[Minchan Kim]

memcg charge/uncharge could be handled by mem_cgroup_[prepare/end]
migration itself so charge local variable in unmap_and_move lost the role
since we introduced 01b1ae63c2.

In addition, the variable name is not good like below.

int unmap_and_move()
{
	charge = mem_cgroup_prepare_migration(xxx);
	..
		BUG_ON(charge); <-- BUG if it is charged?
		..
uncharge:
		if (!charge)    <-- why do we have to uncharge !charge?
			mem_group_end_migration(xxx);
	..
}

So let's remove unnecessary and confusing variable.

Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Suggested-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
---
 mm/memcontrol.c |    1 +
 mm/migrate.c    |    9 +++------
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8a97571..3c91d5c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2873,6 +2873,7 @@ static inline int mem_cgroup_move_swap_account(swp_entry_t entry,
 /*
  * Before starting migration, account PAGE_SIZE to mem_cgroup that the old
  * page belongs to.
+ * Note: Should not return -EAGAIN. unmap_and_move depens on it.
  */
 int mem_cgroup_prepare_migration(struct page *page,
 	struct page *newpage, struct mem_cgroup **ptr, gfp_t gfp_mask)
diff --git a/mm/migrate.c b/mm/migrate.c
index 2abc9c9..37055d0 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -622,7 +622,6 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 	int *result = NULL;
 	struct page *newpage = get_new_page(page, private, &result);
 	int remap_swapcache = 1;
-	int charge = 0;
 	struct mem_cgroup *mem;
 	struct anon_vma *anon_vma = NULL;
 
@@ -637,7 +636,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 		if (unlikely(split_huge_page(page)))
 			goto move_newpage;
 
-	/* prepare cgroup just returns 0 or -ENOMEM */
+	/* mem_cgroup_prepage_migration never returns -EAGAIN */
 	rc = -EAGAIN;
 
 	if (!trylock_page(page)) {
@@ -678,8 +677,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 	}
 
 	/* charge against new page */
-	charge = mem_cgroup_prepare_migration(page, newpage, &mem, GFP_KERNEL);
-	if (charge == -ENOMEM) {
+	if (mem_cgroup_prepare_migration(page, newpage, &mem, GFP_KERNEL)) {
 		rc = -ENOMEM;
 		goto unlock;
 	}
@@ -766,8 +764,7 @@ skip_unmap:
 		drop_anon_vma(anon_vma);
 
 uncharge:
-	if (!charge)
-		mem_cgroup_end_migration(mem, page, newpage, rc == 0);
+	mem_cgroup_end_migration(mem, page, newpage, rc == 0);
 unlock:
 	unlock_page(page);
 
-- 
1.7.1

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Re: [PATCH v2 1/2] memcg: remove unnecessary BUG_ON

[Johannes Weiner]

On Mon, Feb 21, 2011 at 12:17:17AM +0900, Minchan Kim wrote:
> Now memcg in unmap_and_move checks BUG_ON of charge.
> But mem_cgroup_prepare_migration returns either 0 or -ENOMEM.
> If it returns -ENOMEM, it jumps out unlock without the check.
> If it returns 0, it can pass BUG_ON. So it's meaningless.
> Let's remove it.
> 
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
> Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
> ---
>  mm/migrate.c |    1 -
>  1 files changed, 0 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index eb083a6..2abc9c9 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -683,7 +683,6 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  		rc = -ENOMEM;
>  		goto unlock;
>  	}
> -	BUG_ON(charge);

You remove this assertion of the mem_cgroup_prepare_migration() return
value but only add a comment about the expectations in the next patch.

Could you write a full-blown kerneldoc on mem_cgroup_prepare_migration
and remove this BUG_ON() in the same patch?

Re: [PATCH v2 1/2] memcg: remove unnecessary BUG_ON

[Johannes Weiner]

On Mon, Feb 21, 2011 at 12:17:17AM +0900, Minchan Kim wrote:
> Now memcg in unmap_and_move checks BUG_ON of charge.
> But mem_cgroup_prepare_migration returns either 0 or -ENOMEM.
> If it returns -ENOMEM, it jumps out unlock without the check.
> If it returns 0, it can pass BUG_ON. So it's meaningless.
> Let's remove it.
> 
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
> Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
> ---
>  mm/migrate.c |    1 -
>  1 files changed, 0 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index eb083a6..2abc9c9 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -683,7 +683,6 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  		rc = -ENOMEM;
>  		goto unlock;
>  	}
> -	BUG_ON(charge);

You remove this assertion of the mem_cgroup_prepare_migration() return
value but only add a comment about the expectations in the next patch.

Could you write a full-blown kerneldoc on mem_cgroup_prepare_migration
and remove this BUG_ON() in the same patch?

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Re: [PATCH v2 2/2] memcg: remove charge variable in unmap_and_move

[Johannes Weiner]

On Mon, Feb 21, 2011 at 12:17:18AM +0900, Minchan Kim wrote:
> memcg charge/uncharge could be handled by mem_cgroup_[prepare/end]
> migration itself so charge local variable in unmap_and_move lost the role
> since we introduced 01b1ae63c2.
> 
> In addition, the variable name is not good like below.
> 
> int unmap_and_move()
> {
> 	charge = mem_cgroup_prepare_migration(xxx);
> 	..
> 		BUG_ON(charge); <-- BUG if it is charged?
> 		..
> uncharge:
> 		if (!charge)    <-- why do we have to uncharge !charge?
> 			mem_group_end_migration(xxx);
> 	..
> }
> 
> So let's remove unnecessary and confusing variable.
> 
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
> Suggested-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
> ---
>  mm/memcontrol.c |    1 +
>  mm/migrate.c    |    9 +++------
>  2 files changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8a97571..3c91d5c 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2873,6 +2873,7 @@ static inline int mem_cgroup_move_swap_account(swp_entry_t entry,
>  /*
>   * Before starting migration, account PAGE_SIZE to mem_cgroup that the old
>   * page belongs to.
> + * Note: Should not return -EAGAIN. unmap_and_move depens on it.
>   */
>  int mem_cgroup_prepare_migration(struct page *page,
>  	struct page *newpage, struct mem_cgroup **ptr, gfp_t gfp_mask)
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 2abc9c9..37055d0 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -622,7 +622,6 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  	int *result = NULL;
>  	struct page *newpage = get_new_page(page, private, &result);
>  	int remap_swapcache = 1;
> -	int charge = 0;
>  	struct mem_cgroup *mem;
>  	struct anon_vma *anon_vma = NULL;
>  
> @@ -637,7 +636,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  		if (unlikely(split_huge_page(page)))
>  			goto move_newpage;
>  
> -	/* prepare cgroup just returns 0 or -ENOMEM */
> +	/* mem_cgroup_prepage_migration never returns -EAGAIN */
>  	rc = -EAGAIN;

I really don't like this.  Why should we depend on that?

>  	if (!trylock_page(page)) {
> @@ -678,8 +677,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  	}
>  
>  	/* charge against new page */
> -	charge = mem_cgroup_prepare_migration(page, newpage, &mem, GFP_KERNEL);
> -	if (charge == -ENOMEM) {
> +	if (mem_cgroup_prepare_migration(page, newpage, &mem, GFP_KERNEL)) {
>  		rc = -ENOMEM;
>  		goto unlock;

Couldn't we make unmap_and_move completely oblivious of the specific
value and just do

	rc = mem_cgroup_prepare_migration()
	if (rc)
		goto unlock;

at this point?  I think mem_cgroup_prepare_migration should be rather
free to signal pretty much any error and it is up to migrate_pages()
to handle them correctly.

Re: [PATCH v2 2/2] memcg: remove charge variable in unmap_and_move

[Johannes Weiner]

On Mon, Feb 21, 2011 at 12:17:18AM +0900, Minchan Kim wrote:
> memcg charge/uncharge could be handled by mem_cgroup_[prepare/end]
> migration itself so charge local variable in unmap_and_move lost the role
> since we introduced 01b1ae63c2.
> 
> In addition, the variable name is not good like below.
> 
> int unmap_and_move()
> {
> 	charge = mem_cgroup_prepare_migration(xxx);
> 	..
> 		BUG_ON(charge); <-- BUG if it is charged?
> 		..
> uncharge:
> 		if (!charge)    <-- why do we have to uncharge !charge?
> 			mem_group_end_migration(xxx);
> 	..
> }
> 
> So let's remove unnecessary and confusing variable.
> 
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
> Suggested-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
> ---
>  mm/memcontrol.c |    1 +
>  mm/migrate.c    |    9 +++------
>  2 files changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8a97571..3c91d5c 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2873,6 +2873,7 @@ static inline int mem_cgroup_move_swap_account(swp_entry_t entry,
>  /*
>   * Before starting migration, account PAGE_SIZE to mem_cgroup that the old
>   * page belongs to.
> + * Note: Should not return -EAGAIN. unmap_and_move depens on it.
>   */
>  int mem_cgroup_prepare_migration(struct page *page,
>  	struct page *newpage, struct mem_cgroup **ptr, gfp_t gfp_mask)
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 2abc9c9..37055d0 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -622,7 +622,6 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  	int *result = NULL;
>  	struct page *newpage = get_new_page(page, private, &result);
>  	int remap_swapcache = 1;
> -	int charge = 0;
>  	struct mem_cgroup *mem;
>  	struct anon_vma *anon_vma = NULL;
>  
> @@ -637,7 +636,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  		if (unlikely(split_huge_page(page)))
>  			goto move_newpage;
>  
> -	/* prepare cgroup just returns 0 or -ENOMEM */
> +	/* mem_cgroup_prepage_migration never returns -EAGAIN */
>  	rc = -EAGAIN;

I really don't like this.  Why should we depend on that?

>  	if (!trylock_page(page)) {
> @@ -678,8 +677,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  	}
>  
>  	/* charge against new page */
> -	charge = mem_cgroup_prepare_migration(page, newpage, &mem, GFP_KERNEL);
> -	if (charge == -ENOMEM) {
> +	if (mem_cgroup_prepare_migration(page, newpage, &mem, GFP_KERNEL)) {
>  		rc = -ENOMEM;
>  		goto unlock;

Couldn't we make unmap_and_move completely oblivious of the specific
value and just do

	rc = mem_cgroup_prepare_migration()
	if (rc)
		goto unlock;

at this point?  I think mem_cgroup_prepare_migration should be rather
free to signal pretty much any error and it is up to migrate_pages()
to handle them correctly.

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Re: [PATCH v2 1/2] memcg: remove unnecessary BUG_ON

[Minchan Kim]

On Mon, Feb 21, 2011 at 10:04 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> On Mon, Feb 21, 2011 at 12:17:17AM +0900, Minchan Kim wrote:
>> Now memcg in unmap_and_move checks BUG_ON of charge.
>> But mem_cgroup_prepare_migration returns either 0 or -ENOMEM.
>> If it returns -ENOMEM, it jumps out unlock without the check.
>> If it returns 0, it can pass BUG_ON. So it's meaningless.
>> Let's remove it.
>>
>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>> Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
>> Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
>> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>> Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
>> ---
>>  mm/migrate.c |    1 -
>>  1 files changed, 0 insertions(+), 1 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index eb083a6..2abc9c9 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -683,7 +683,6 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>>               rc = -ENOMEM;
>>               goto unlock;
>>       }
>> -     BUG_ON(charge);
>
> You remove this assertion of the mem_cgroup_prepare_migration() return
> value but only add a comment about the expectations in the next patch.
>
> Could you write a full-blown kerneldoc on mem_cgroup_prepare_migration
> and remove this BUG_ON() in the same patch?
>

Okay. I could.




-- 
Kind regards,
Minchan Kim

Re: [PATCH v2 1/2] memcg: remove unnecessary BUG_ON

[Minchan Kim]

On Mon, Feb 21, 2011 at 10:04 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> On Mon, Feb 21, 2011 at 12:17:17AM +0900, Minchan Kim wrote:
>> Now memcg in unmap_and_move checks BUG_ON of charge.
>> But mem_cgroup_prepare_migration returns either 0 or -ENOMEM.
>> If it returns -ENOMEM, it jumps out unlock without the check.
>> If it returns 0, it can pass BUG_ON. So it's meaningless.
>> Let's remove it.
>>
>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>> Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
>> Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
>> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>> Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
>> ---
>>  mm/migrate.c |    1 -
>>  1 files changed, 0 insertions(+), 1 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index eb083a6..2abc9c9 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -683,7 +683,6 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>>               rc = -ENOMEM;
>>               goto unlock;
>>       }
>> -     BUG_ON(charge);
>
> You remove this assertion of the mem_cgroup_prepare_migration() return
> value but only add a comment about the expectations in the next patch.
>
> Could you write a full-blown kerneldoc on mem_cgroup_prepare_migration
> and remove this BUG_ON() in the same patch?
>

Okay. I could.




-- 
Kind regards,
Minchan Kim

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