Re: [PATCH] zram: support REQ_DISCARD

[Joonsoo Kim]

2014-02-25 1:06 GMT+09:00 Jerome Marchand <jmarchan@xxxxxxxxxx>:
> On 02/24/2014 04:56 PM, Joonsoo Kim wrote:
>> 2014-02-25 0:15 GMT+09:00 Jerome Marchand <jmarchan@xxxxxxxxxx>:
>>> On 02/24/2014 04:02 PM, Joonsoo Kim wrote:
>>>> 2014-02-24 22:36 GMT+09:00 Jerome Marchand <jmarchan@xxxxxxxxxx>:
>>>>> On 02/24/2014 06:51 AM, Joonsoo Kim wrote:
>>>>>> zram is ram based block device and can be used by backend of filesystem.
>>>>>> When filesystem deletes a file, it normally doesn't do anything on data
>>>>>> block of that file. It just marks on metadata of that file. This behavior
>>>>>> has no problem on disk based block device, but has problems on ram based
>>>>>> block device, since we can't free memory used for data block. To overcome
>>>>>> this disadvantage, there is REQ_DISCARD functionality. If block device
>>>>>> support REQ_DISCARD and filesystem is mounted with discard option,
>>>>>> filesystem sends REQ_DISCARD to block device whenever some data blocks are
>>>>>> discarded. All we have to do is to handle this request.
>>>>>>
>>>>>> This patch implements to flag up QUEUE_FLAG_DISCARD and handle this
>>>>>> REQ_DISCARD request. With it, we can free memory used by zram if it isn't
>>>>>> used.
>>>>>>
>>>>>> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
>>>>>> ---
>>>>>> This patch is based on master branch of linux-next tree.
>>>>>>
>>>>>> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
>>>>>> index 5ec61be..cff2c0e 100644
>>>>>> --- a/drivers/block/zram/zram_drv.c
>>>>>> +++ b/drivers/block/zram/zram_drv.c
>>>>>> @@ -501,6 +501,20 @@ static int zram_bvec_rw(struct zram *zram, struct bio_vec *bvec, u32 index,
>>>>>>       return ret;
>>>>>>  }
>>>>>>
>>>>>> +static void zram_bio_discard(struct zram *zram, struct bio *bio)
>>>>>> +{
>>>>>> +     u32 index = bio->bi_iter.bi_sector >> SECTORS_PER_PAGE_SHIFT;
>>>>>
>>>>> Hi Joonsoo,
>>>>>
>>>>> If bi_sector is not aligned on a page size, we might end up discarding
>>>>> a page that still contain valid data.
>>>>>
>>>>>
>>>>
>>>> Hello, Jerome.
>>>>
>>>> Is it possible that request isn't aligned on a page size if
>>>> logical/physical block size
>>>> is PAGE_SIZE?
>>>
>>> Yes, zram has an logical block size of 4k (ZRAM_LOGICAL_BLOCK_SIZE),
>>> while its physical block size, which is a page size, can be bigger.
>>>
>>>> When I tested it, I didn't find any invalid io.
>>>> If we meet any misaligned request, it would be filtered by
>>>> valid_io_request(). :)
>>>
>>> zram accepts request aligned on logical blocks. So valid_io_request()
>>> wouldn't filter misaligned requests out as long as they are aligned
>>> on logical blocks.
>>> If your system use 4k pages, your tests would never trigger the issue,
>>> but on a system which uses 64k pages, it could.
>>
>> Okay. I got it.
>> So, how about using PAGE_SIZE as ZRAM_LOGICAL_BLOCK_SIZE?
>> Is there any reason to set 4096 to ZRAM_LOGICAL_BLOCK_SIZE,
>> instead of setting PAGE_SIZE to ZRAM_LOGICAL_BLOCK_SIZE?
>>
>
> ZRAM_LOGICAL_BLOCK_SIZE was introduced in commit 7b19b8d because the
> block layer couldn't handle 64k logical block. Also, some filesytems
> (including FAT IRC), can't cope with 64k block either.
>

Okay. I will check it more.

Thanks for nice comment!!
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