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

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Date:   Wed, 9 Aug 2017 20:06:24 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        "karam . lee" <karam.lee@....com>, seungho1.park@....com,
        Christoph Hellwig <hch@....de>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        Jens Axboe <axboe@...nel.dk>,
        Vishal Verma <vishal.l.verma@...el.com>,
        "linux-nvdimm@...ts.01.org" <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 Wed, Aug 9, 2017 at 8:04 PM, Matthew Wilcox <willy@...radead.org> 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 like it, but do you think we should switch to sbvec[<constant>] to
preclude pathological cases where nr_pages is large?