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

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Message-ID: <20170811104615.GA14397@lst.de>
Date:   Fri, 11 Aug 2017 12:46:15 +0200
From:   Christoph Hellwig <hch@....de>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        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 09, 2017 at 08:06:24PM -0700, Dan Williams wrote:
> I like it, but do you think we should switch to sbvec[<constant>] to
> preclude pathological cases where nr_pages is large?

Yes, please.

Then I'd like to see that the on-stack bio even matters for
mpage_readpage / mpage_writepage.  Compared to all the buffer head
overhead the bio allocation should not actually matter in practice.