linux-kernel - Re: [PATCH] block: respect virtual boundary mask in bvecs

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Message-ID: <1541607019.196084.210.camel@acm.org>
Date:   Wed, 07 Nov 2018 08:10:19 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     Sagi Grimberg <sagi@...mberg.me>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Jens Axboe <axboe@...com>
Cc:     Linux Block Layer Mailinglist <linux-block@...r.kernel.org>,
        Hannes Reinecke <hare@...e.de>,
        Linux Kernel Mailinglist <linux-kernel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] block: respect virtual boundary mask in bvecs

On Tue, 2018-11-06 at 19:30 -0800, Sagi Grimberg wrote:
> > > How about changing that expression into the following to make it easier
> > > for the compiler to optimize this code?
> > > 
> > > (offset | (bprv->bv_offset + bprv->bv_len)) & queue_virt_boundary(q)
> > 
> > Uhm I have to admit I'm not really able to parse the above expression.
> > Sure GCC will do it but I think it's less readable (at least for me).
> > Let's see what other's think.
> 
> I personally not a huge fan of decoding complicated expressions. But if
> others are fine with it then I am too...

What I proposed is not a new pattern. It is a pattern that is already used
elsewhere in the Linux kernel. A few examples:

>From dmabounce.c:

		/* Figure out if we need to bounce from the DMA mask. */
		if ((dma_addr | (dma_addr + size - 1)) & ~mask)
			return 1;

>From dma-direct.h:

	if ((addr | (addr + size - 1)) & ~mask)
		return 0;

Bart.