linux-kernel - Re: [PATCH 4.19 01/16] block: introduce multi-page bvec helpers

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Date:   Thu, 11 Nov 2021 09:08:47 -0800
From:   Zubin Mithra <zsm@...omium.org>
To:     Pavel Machek <pavel@....cz>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Christoph Hellwig <hch@....de>, Omar Sandoval <osandov@...com>,
        Ming Lei <ming.lei@...hat.com>, Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH 4.19 01/16] block: introduce multi-page bvec helpers

On Thu, Nov 11, 2021 at 05:47:54PM +0100, Pavel Machek wrote:
> Hi!
> 
> > From: Ming Lei <ming.lei@...hat.com>
> > 
> > commit 3d75ca0adef4280650c6690a0c4702a74a6f3c95 upstream.
> > 
> > This patch introduces helpers of 'mp_bvec_iter_*' for multi-page bvec
> > support.
> > 
> > The introduced helpers treate one bvec as real multi-page segment,
> > which may include more than one pages.
> > 
> > The existed helpers of bvec_iter_* are interfaces for supporting current
> > bvec iterator which is thought as single-page by drivers, fs, dm and
> > etc. These introduced helpers will build single-page bvec in flight, so
> > this way won't break current bio/bvec users, which needn't any
> > change.
> 
> I don't understand why we have this in 4.19-stable. I don't see
> followup patches needing it, and it does not claim to fix a bug.


There is some more context on this at:
https://lore.kernel.org/linux-block/YXweJ00CVsDLCI7b@google.com/T/#u
and
https://lore.kernel.org/stable/YYVZBuDaWBKT3vOS@google.com/T/#u

Thanks,
- Zubin

> 
> > +#define mp_bvec_iter_bvec(bvec, iter)				\
> > +((struct bio_vec) {						\
> > +	.bv_page	= mp_bvec_iter_page((bvec), (iter)),	\
> > +	.bv_len		= mp_bvec_iter_len((bvec), (iter)),	\
> > +	.bv_offset	= mp_bvec_iter_offset((bvec), (iter)),	\
> > +})
> > +
> > +/* For building single-page bvec in flight */
> > + #define bvec_iter_offset(bvec, iter)				\
> > +	(mp_bvec_iter_offset((bvec), (iter)) % PAGE_SIZE)
> > +
> 
> Plus this one is strange. IIRC preprocessor directives have to put #
> in column zero?
> 
> Best regards,
> 								Pavel
> -- 
> http://www.livejournal.com/~pavelmachek