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SubjectRe: [PATCH] zram: support REQ_DISCARD
2014-02-25 0:15 GMT+09:00 Jerome Marchand <>:
> On 02/24/2014 04:02 PM, Joonsoo Kim wrote:
>> 2014-02-24 22:36 GMT+09:00 Jerome Marchand <>:
>>> 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 <>
>>>> ---
>>>> 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.
Is there any reason to set 4096 to ZRAM_LOGICAL_BLOCK_SIZE,
instead of setting PAGE_SIZE to ZRAM_LOGICAL_BLOCK_SIZE?


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