Re: [PATCH v2 17/21] block, blksnap: snapshot image block device

From: Sergei Shtepa <sergei.shtepa@veeam.com>
To: Hillf Danton <hdanton@sina.com>
Cc: "axboe@kernel.dk" <axboe@kernel.dk>,
	"corbet@lwn.net" <corbet@lwn.net>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 17/21] block, blksnap: snapshot image block device
Date: Mon, 2 Jan 2023 10:44:06 +0100	[thread overview]
Message-ID: <b2ec41dd-3346-c2db-d59c-9050f8ad1053@veeam.com> (raw)
In-Reply-To: <20230101071813.3329-1-hdanton@sina.com>

On 1/1/23 08:18, Hillf Danton wrote:
> Subject:
> Re: [PATCH v2 17/21] block, blksnap: snapshot image block device
> From:
> Hillf Danton <hdanton@sina.com>
> Date:
> 1/1/23, 08:18
> 
> To:
> Sergei Shtepa <sergei.shtepa@veeam.com>
> CC:
> "axboe@kernel.dk" <axboe@kernel.dk>, "corbet@lwn.net" <corbet@lwn.net>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
> 
> 
> This is the first time you've received an email from this sender 
> hdanton@sina.com, please exercise caution when clicking on links or opening 
> attachments.
> 
> 
> On 9 Dec 2022 15:23:27 +0100 Sergei Shtepa
>  > Provides the operation of block devices of snapshot images. Read and
>  > write operations are redirected to the regions of difference blocks for
>  > block device (struct diff_area).
>  >
>  > Signed-off-by: Sergei Shtepa
>  > ---
> 
> Thanks for your patchset.

Thanks for the review.

> 
>  > +static int snapimage_kthread_worker_fn(void *param)
>  > +{
>  > + struct snapimage *snapimage = param;
>  > + struct bio *bio;
>  > + int ret = 0;
>  > +
>  > + while (!kthread_should_stop()) {
>  > + bio = get_bio_from_queue(snapimage);
>  > + if (!bio) {
>  > + schedule_timeout_interruptible(HZ / 100);
>  > + continue;
>  > + }
> 
> Given the wake_up_process() below, s$HZ / 100$HZ * 1000$ to avoid
> unnecessary wakeups [1].
> 
> And because of no signal handling added, use schedule_timeout_idle() instead.

Yes, this function will be rewritten to eliminate unnecessary wake-ups.
The code that Christoph proposed in the letter at Dec 15 should work well.

> 
> [1] 
> https://nam10.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flkml%2F20210419085027.761150-2-elver%40google.com%2F&data=05%7C01%7Csergei.shtepa%40veeam.com%7C765d591d47ad4d93857208daebc92508%7Cba07baab431b49edadd7cbc3542f5140%7C1%7C1%7C638081546440884849%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=rbINOq7m31DFhjHLnQKFHQuoWB64ZhmnWbAVLuNwv7w%3D&reserved=0
> 
>  > +
>  > + snapimage_process_bio(snapimage, bio);
>  > + }
>  > +
>  > + while ((bio = get_bio_from_queue(snapimage)))
>  > + snapimage_process_bio(snapimage, bio);
>  > +
>  > + return ret;
>  > +}
>  > +
>  > +static void snapimage_submit_bio(struct bio *bio)
>  > +{
>  > + struct snapimage *snapimage = bio->bi_bdev->bd_disk->private_data;
>  > + gfp_t gfp = GFP_NOIO;
>  > +
>  > + if (bio->bi_opf & REQ_NOWAIT)
>  > + gfp |= GFP_NOWAIT;
>  > + if (snapimage->is_ready) {
>  > + spin_lock(&snapimage->queue_lock);
>  > + bio_list_add(&snapimage->queue, bio);
>  > + spin_unlock(&snapimage->queue_lock);
>  > +
>  > + wake_up_process(snapimage->worker);
>  > + } else
>  > + bio_io_error(bio);
>  > +}
> 