linux-kernel - Re: [PATCH v1 2/6] fs: use on-stack-bio if backing device has BDI_CAP

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Message-ID: <20170809024150.GA32471@bbox>
Date:   Wed, 9 Aug 2017 11:41:50 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        "karam . lee" <karam.lee@....com>, seungho1.park@....com,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>, jack@...e.cz,
        Jens Axboe <axboe@...nel.dk>,
        Vishal Verma <vishal.l.verma@...el.com>,
        linux-nvdimm@...ts.01.org, kernel-team <kernel-team@....com>
Subject: Re: [PATCH v1 2/6] fs: use on-stack-bio if backing device has
 BDI_CAP_SYNC capability

On Tue, Aug 08, 2017 at 07:31:22PM -0700, Matthew Wilcox wrote:
> On Wed, Aug 09, 2017 at 10:51:13AM +0900, Minchan Kim wrote:
> > On Tue, Aug 08, 2017 at 06:29:04AM -0700, Matthew Wilcox wrote:
> > > On Tue, Aug 08, 2017 at 05:49:59AM -0700, Matthew Wilcox wrote:
> > > > +	struct bio sbio;
> > > > +	struct bio_vec sbvec;
> > > 
> > > ... this needs to be sbvec[nr_pages], of course.
> > > 
> > > > -		bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
> > > > +		if (bdi_cap_synchronous_io(inode_to_bdi(inode))) {
> > > > +			bio = &sbio;
> > > > +			bio_init(bio, &sbvec, nr_pages);
> > > 
> > > ... and this needs to be 'sbvec', not '&sbvec'.
> > 
> > I don't get it why we need sbvec[nr_pages].
> > On-stack-bio works with per-page.
> > May I miss something?
> 
> The way I redid it, it will work with an arbitrary number of pages.

IIUC, it would be good things with dynamic bio alloction with passing
allocated bio back and forth but on-stack bio cannot work like that.
It should be done in per-page so it is worth?