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

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Date:	Tue, 10 Aug 2010 11:54:35 -0400
From:	Jens Axboe <jaxboe@...ionio.com>
To:	"ngupta@...are.org" <ngupta@...are.org>
CC:	Pekka Enberg <penberg@...nel.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 08/10/2010 12:54 AM, Nitin Gupta wrote:
> On 08/10/2010 07:53 AM, Jens Axboe wrote:
>> 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?
> 
> For reads on freed/unwritten sectors, it simply returns success and
> does not touch the bio page. Is it better to zero the page in such
> cases?

Well, you told the kernel that you return zeroes on discarded ranges:

        zram->disk->queue->limits.discard_zeroes_data = 1;

So yes, if you intend to keep that, then you need to zero the
incoming pages that have been explicitly trimmed by a discard.

-- 
Jens Axboe


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