On 05/07/2010 02:52 PM, Nigel Cunningham wrote:
> Hi.
>
> On 07/05/10 17:25, Nitin Gupta wrote:
>> This callback is required when RAM based devices are used as swap disks.
>> One such device is ramzswap which is used as compressed in-memory swap
>> disk. For such devices, we need a callback as soon as a swap slot is no
>> longer used to allow freeing memory allocated for this slot. Without
>> this
>> callback, stale data can quickly accumulate in memory defeating the whole
>> purpose of such devices.
>>
>> Signed-off-by: Nitin Gupta<ngupta@xxxxxxxxxx>
>> ---
>> include/linux/blkdev.h | 2 ++
>> mm/swapfile.c | 4 ++++
>> 2 files changed, 6 insertions(+), 0 deletions(-)
>>
>> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
>> index 6690e8b..413284a 100644
>> --- a/include/linux/blkdev.h
>> +++ b/include/linux/blkdev.h
>> @@ -1287,6 +1287,8 @@ 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 sometimes page table lock
>> held */
>> + void (*swap_slot_free_notify) (struct block_device *, unsigned
>> long);
>> struct module *owner;
>> };
>>
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index ecb069e..f5ccc47 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -574,6 +574,7 @@ static unsigned char swap_entry_free(struct
>> swap_info_struct *p,
>>
>> /* free if no reference */
>> if (!usage) {
>> + struct gendisk *disk = p->bdev->bd_disk;
>> if (offset< p->lowest_bit)
>> p->lowest_bit = offset;
>> if (offset> p->highest_bit)
>> @@ -583,6 +584,9 @@ static unsigned char swap_entry_free(struct
>> swap_info_struct *p,
>> swap_list.next = p->type;
>> nr_swap_pages++;
>> p->inuse_pages--;
>> + if ((p->flags& SWP_BLKDEV)&&
>> + disk->fops->swap_slot_free_notify)
>> + disk->fops->swap_slot_free_notify(p->bdev, offset);
>
> Is this p->flags & SWP_BLKDEV logic reversed? (Don't you want the
> notifier called for devices that aren't backed by a block device?)
>
No, the logic here is correct: ramzswap is a block device for which
we want this callback. Though its a RAM backed, it is still a block
device.
(I hope it answers your question in the other mail also).
> I also wonder whether leaving the p->flags & SWP_BLKDEV part out might
> be a good idea. Other potential notifier users?
>
For regular files, 'offset' used here makes little sense. For block devices,
its simply offset in real device. Also, I doubt if *files* would ever
like to have such a callback.
Thanks,
Nitin
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