linux-kernel - Re: [PATCH 06/10] Block discard support

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Date:	Mon, 9 Aug 2010 22:23:39 -0400
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Pekka Enberg <penberg@...nel.org>
CC:	Nitin Gupta <ngupta@...are.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Minchan Kim <minchan.kim@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg KH <greg@...ah.com>,
	Linux Driver Project <devel@...uxdriverproject.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 06/10] Block discard support

On 08/09/2010 03:03 PM, Pekka Enberg wrote:
> On Mon, Aug 9, 2010 at 8:26 PM, Nitin Gupta <ngupta@...are.org> wrote:
>> The 'discard' bio discard request provides information to
>> zram disks regarding blocks which are no longer in use by
>> filesystem. This allows freeing memory allocated for such
>> blocks.
>>
>> When zram devices are used as swap disks, we already have
>> a callback (block_device_operations->swap_slot_free_notify).
>> So, the discard support is useful only when used as generic
>> (non-swap) disk.
>>
>> Signed-off-by: Nitin Gupta <ngupta@...are.org>
> 
> Lets CC fsdevel and Jens for this.

Looks OK from a quick look. One comment, though:

>> +static void zram_discard(struct zram *zram, struct bio *bio)
>> +{
>> +       size_t bytes = bio->bi_size;
>> +       sector_t sector = bio->bi_sector;
>> +
>> +       while (bytes >= PAGE_SIZE) {
>> +               zram_free_page(zram, sector >> SECTORS_PER_PAGE_SHIFT);
>> +               sector += PAGE_SIZE >> SECTOR_SHIFT;
>> +               bytes -= PAGE_SIZE;
>> +       }
>> +
>> +       bio_endio(bio, 0);
>> +}
>> +

So freeing the page here will guarantee zeroed return on read?
And since you set PAGE_SIZE as the discard granularity, the above loop
could be coded more readable with the knowledge that ->bi_size is always
a multiple of the page size.

-- 
Jens Axboe


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