Date: Thu, 10 Aug 2017 13:00:14 +0900
From: Minchan Kim <minchan@kernel.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
        "karam . lee" <karam.lee@lge.com>, seungho1.park@lge.com,
        Christoph Hellwig <hch@lst.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Chinner <david@fromorbit.com>, jack@suse.cz,
        Jens Axboe <axboe@kernel.dk>, Vishal Verma <vishal.l.verma@intel.com>,
        linux-nvdimm@lists.01.org, kernel-team <kernel-team@lge.com>
Subject: Re: [PATCH v1 2/6] fs: use on-stack-bio if backing device has
 BDI_CAP_SYNC capability
Message-ID: <20170810040014.GA2042@bbox>
References: <1502175024-28338-1-git-send-email-minchan@kernel.org>
 <1502175024-28338-3-git-send-email-minchan@kernel.org>
 <20170808124959.GB31390@bombadil.infradead.org>
 <20170808132904.GC31390@bombadil.infradead.org>
 <20170809015113.GB32338@bbox>
 <20170809023122.GF31390@bombadil.infradead.org>
 <20170809024150.GA32471@bbox>
 <20170810030433.GG31390@bombadil.infradead.org>
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On Wed, Aug 09, 2017 at 08:04:33PM -0700, Matthew Wilcox wrote:
> On Wed, Aug 09, 2017 at 11:41:50AM +0900, Minchan Kim wrote:
> > 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?
> 
> I'm not passing the bio back and forth between do_mpage_readpage() and
> its callers.  The version I sent allows for multiple pages in a single
> on-stack bio (when called from mpage_readpages()).

I'm confused. I want to confirm your thought before respinning.
Please correct me if I miss something.

The version you sent to me used on-stack bio within do_mpage_readpage
so that's why I said sbvec[nr_pages] would be pointless because it
works with per-page base unless if we use dynamic bio allocation.

But I guess now you suggest to use on-stack bio in mpage_readpages so
single on-stack bio in mpage_readpages's stack can batch multiple pages
in bvecs of a bio.

Right?