linux-kernel - Re: [PATCH V4 1/8] null_blk: allow REQ_OP_WRITE

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Message-ID: <bebfa1a2-1bf6-b515-00fb-4a7be7eeeae5@nvidia.com>
Date:   Wed, 30 Nov 2022 23:29:00 +0000
From:   Chaitanya Kulkarni <chaitanyak@...dia.com>
To:     Shinichiro Kawasaki <shinichiro.kawasaki@....com>,
        Chaitanya Kulkarni <chaitanyak@...dia.com>
CC:     "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "damien.lemoal@...nsource.wdc.com" <damien.lemoal@...nsource.wdc.com>,
        "bvanassche@....org" <bvanassche@....org>,
        "vincent.fu@...sung.com" <vincent.fu@...sung.com>
Subject: Re: [PATCH V4 1/8] null_blk: allow REQ_OP_WRITE_ZEROES

>> Signed-off-by: Chaitanya Kulkarni <kch@...dia.com>
> 
> This motivation sounds good. I tried this patch. With a quick test it looks
> working good for me. Please find minor comments in line.
> 
> [...]
> 
>> +static void null_zero_sector(struct nullb_device *d, sector_t sect,
>> +			     sector_t nr_sects, bool cache)
>> +{
>> +	struct radix_tree_root *root = cache ? &d->cache : &d->data;
>> +	struct nullb_page *t_page;
>> +	unsigned int offset;
>> +	void *dest;
>> +
>> +	t_page = radix_tree_lookup(root, sect >> PAGE_SECTORS_SHIFT);
>> +	if (!t_page)
>> +		return;
>> +
>> +	offset = (sect & SECTOR_MASK) << SECTOR_SHIFT;
>> +	dest = kmap_atomic(t_page->page);
>> +	memset(dest + offset, 0, SECTOR_SIZE * nr_sects);
>> +	kunmap_atomic(dest);
>> +}
> 
> Did you consider to call null_lookup_page() for __null_lookup_page() from
> null_zero_sector()? It may simplify this function a bit.
> 

I found this clear and easy than going over call chain when
debugging.

>> +
>>   static struct nullb_page *null_radix_tree_insert(struct nullb *nullb, u64 idx,

[...]

>>   
>> +static void null_config_write_zeroes(struct nullb *nullb)
>> +{
>> +	if (!nullb->dev->write_zeroes)
>> +		return;
>> +	blk_queue_max_write_zeroes_sectors(nullb->q, UINT_MAX >> 9);
> 
> Just comment: this value UINT_MAX >> 9 sounds a bit weird, but probably ok. This
> value was introduced by commit 306eb6b4ad4f ("nullb: support discard") to call
> blk_queue_max_discard_sectors(). I guess you chose the same value for write
> zeroes.
> 

Yes indeed, plz have a look end patches to allow user to set this value.

-ck