[PATCH] [mmotm] comment on swap notify locking constraints

[PATCH] [mmotm] comment on swap notify locking constraints

[Nitin Gupta]

A block device makes swap_slot_free_notify() callback
when the last reference to a swap slot is dropped.

This callback is made under swap_lock and often with
lock for corresponding swapcache page also held. This
is a note of warning for registered callback function
which must meet these constraints.

Signed-off-by: Nitin Gupta <ngupta@vflare.org>
---
 include/linux/blkdev.h |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 14b95a3..e3dcff6 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1295,6 +1295,7 @@ struct block_device_operations {
 						unsigned long long);
 	int (*revalidate_disk) (struct gendisk *);
 	int (*getgeo)(struct block_device *, struct hd_geometry *);
+	/* this callback is with swap_lock and often page lock also held */
 	void (*swap_slot_free_notify) (struct block_device *, unsigned long);
 	struct module *owner;
 };
-- 
1.6.2.5

Re: [PATCH] [mmotm] comment on swap notify locking constraints

[Hugh Dickins]

On Thu, 14 Jan 2010, Nitin Gupta wrote:

> A block device makes swap_slot_free_notify() callback
> when the last reference to a swap slot is dropped.
> 
> This callback is made under swap_lock and often with
> lock for corresponding swapcache page also held. This
> is a note of warning for registered callback function
> which must meet these constraints.

You're right that the page lock of the corresponding swapcache page
may be held there, but that isn't what worried me enough to ask for
a comment: it's the page table lock (sometimes we call it pte lock),
a spinlock like the swap_lock, being held there that particularly
deserves comment.  We don't want someone going to lots of effort
to avoid holding swap_lock there, only to find that effort wasted
because page table lock is also held.

Hugh

> 
> Signed-off-by: Nitin Gupta <ngupta@vflare.org>
> ---
>  include/linux/blkdev.h |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 14b95a3..e3dcff6 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1295,6 +1295,7 @@ struct block_device_operations {
>  						unsigned long long);
>  	int (*revalidate_disk) (struct gendisk *);
>  	int (*getgeo)(struct block_device *, struct hd_geometry *);
> +	/* this callback is with swap_lock and often page lock also held */

                                                     and page table lock

>  	void (*swap_slot_free_notify) (struct block_device *, unsigned long);
>  	struct module *owner;
>  };
> -- 
> 1.6.2.5

Re: [PATCH] [mmotm] comment on swap notify locking constraints

[Nitin Gupta]

On 01/14/2010 08:40 PM, Hugh Dickins wrote:
> On Thu, 14 Jan 2010, Nitin Gupta wrote:
> 
>> A block device makes swap_slot_free_notify() callback
>> when the last reference to a swap slot is dropped.
>>
>> This callback is made under swap_lock and often with
>> lock for corresponding swapcache page also held. This
>> is a note of warning for registered callback function 
>> which must meet these constraints.
> 
> You're right that the page lock of the corresponding swapcache page
> may be held there, but that isn't what worried me enough to ask for
> a comment: it's the page table lock (sometimes we call it pte lock),
> a spinlock like the swap_lock, being held there that particularly
> deserves comment.  We don't want someone going to lots of effort
> to avoid holding swap_lock there, only to find that effort wasted
> because page table lock is also held.

Thanks for the review.

I was missing this important point regarding pte lock. I will edit
the comment and resend the patch.

Nitin





>>
>> Signed-off-by: Nitin Gupta <ngupta@vflare.org>
>> ---
>>  include/linux/blkdev.h |    1 +
>>  1 files changed, 1 insertions(+), 0 deletions(-)
>>
>> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
>> index 14b95a3..e3dcff6 100644
>> --- a/include/linux/blkdev.h
>> +++ b/include/linux/blkdev.h
>> @@ -1295,6 +1295,7 @@ struct block_device_operations {
>>  						unsigned long long);
>>  	int (*revalidate_disk) (struct gendisk *);
>>  	int (*getgeo)(struct block_device *, struct hd_geometry *);
>> +	/* this callback is with swap_lock and often page lock also held */
> 
>                                                      and page table lock
> 
>>  	void (*swap_slot_free_notify) (struct block_device *, unsigned long);
>>  	struct module *owner;
>>  };
>> -- 
>> 1.6.2.5
>