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SubjectRe: [PATCH v2] zram: support REQ_DISCARD
From
2014-02-26 17:07 GMT+09:00 Minchan Kim <minchan@kernel.org>:
> Hi Joonsoo,
>
> On Wed, Feb 26, 2014 at 02:23:15PM +0900, 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.
>>
>> v2: handle unaligned case commented by Jerome
>>
>> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>>
>> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
>> index 5ec61be..5364c1e 100644
>> --- a/drivers/block/zram/zram_drv.c
>> +++ b/drivers/block/zram/zram_drv.c
>> @@ -501,6 +501,36 @@ 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;
>> + size_t n = bio->bi_iter.bi_size;
>
> Nitpick:
> Please use more meaningful name(ex, len) rather than 'n'.
>

Hello, Minchan.

Will do.

>> + size_t misalign;
>> +
>> + * On some arch, logical block (4096) aligned request couldn't be
>> + * aligned to PAGE_SIZE, since their PAGE_SIZE aren't 4096.
>> + * Therefore we should handle this misaligned case here.
>> + */
>> + misalign = (bio->bi_iter.bi_sector &
>> + (SECTORS_PER_PAGE - 1)) << SECTOR_SHIFT;
>> + if (misalign) {
>> + if (n < misalign)
>> + return;
>> +
>> + n -= misalign;
>> + index++;
>> + }
>> +
>> + while (n >= PAGE_SIZE) {
>> + write_lock(&zram->meta->tb_lock);
>> + zram_free_page(zram, index);
>> + write_unlock(&zram->meta->tb_lock);
>> + index++;
>> + n -= PAGE_SIZE;
>> + }
>> +}
>> +
>> static void zram_reset_device(struct zram *zram, bool reset_capacity)
>> {
>> size_t index;
>> @@ -618,6 +648,12 @@ static void __zram_make_request(struct zram *zram, struct bio *bio)
>> struct bio_vec bvec;
>> struct bvec_iter iter;
>>
>> + if (unlikely(bio->bi_rw & REQ_DISCARD)) {
>> + zram_bio_discard(zram, bio);
>> + bio_endio(bio, 0);
>> + return;
>> + }
>> +
>> index = bio->bi_iter.bi_sector >> SECTORS_PER_PAGE_SHIFT;
>> offset = (bio->bi_iter.bi_sector &
>> (SECTORS_PER_PAGE - 1)) << SECTOR_SHIFT;
>> @@ -784,6 +820,10 @@ static int create_device(struct zram *zram, int device_id)
>> ZRAM_LOGICAL_BLOCK_SIZE);
>> blk_queue_io_min(zram->disk->queue, PAGE_SIZE);
>> blk_queue_io_opt(zram->disk->queue, PAGE_SIZE);
>> + zram->disk->queue->limits.discard_granularity = PAGE_SIZE;
>> + zram->disk->queue->limits.max_discard_sectors = UINT_MAX;
>> + zram->disk->queue->limits.discard_zeroes_data = 1;
>
> I don't know what discard_zeroes_data does mean. It seems we should
> make sure zram should return zero pages for discarded block on next
> time but prolblem could happen if you bail out in discard logic
> due to misalign but caller seem to know it was successful?
>
> What happens in this case?
>

This will result in the problem what you think about.
I will change it like as following.

if (PAGE_SIZE == ZRAM_LOGICAL_BLOCK_SIZE)
zram->disk->queue->limits.discard_zeroes_data = 1;
else
zram->disk->queue->limits.discard_zeroes_data = 0;

Does It work for you?

Thanks.


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