linux-ext4 - Re: don't use ->bd_inode to access the block device size v3

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Date:   Mon, 18 Oct 2021 19:18:43 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Christoph Hellwig <hch@....de>, Coly Li <colyli@...e.de>,
        Mike Snitzer <snitzer@...hat.com>, Song Liu <song@...nel.org>,
        David Sterba <dsterba@...e.com>,
        Josef Bacik <josef@...icpanda.com>,
        Theodore Ts'o <tytso@....edu>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        Dave Kleikamp <shaggy@...nel.org>,
        Ryusuke Konishi <konishi.ryusuke@...il.com>,
        Anton Altaparmakov <anton@...era.com>,
        Konstantin Komarov <almaz.alexandrovich@...agon-software.com>,
        Kees Cook <keescook@...omium.org>,
        Phillip Lougher <phillip@...ashfs.org.uk>,
        Jan Kara <jack@...e.com>, linux-block@...r.kernel.org,
        dm-devel@...hat.com, drbd-dev@...ts.linbit.com,
        linux-bcache@...r.kernel.org, linux-raid@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-scsi@...r.kernel.org,
        target-devel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-ext4@...r.kernel.org,
        jfs-discussion@...ts.sourceforge.net, linux-nfs@...r.kernel.org,
        linux-nilfs@...r.kernel.org, linux-ntfs-dev@...ts.sourceforge.net,
        ntfs3@...ts.linux.dev, reiserfs-devel@...r.kernel.org
Subject: Re: don't use ->bd_inode to access the block device size v3

On Mon, Oct 18, 2021 at 11:16:08AM -0600, Jens Axboe wrote:
> This looks good to me. Followup question, as it's related - I've got a
> hacky patch that caches the inode size in the bdev:
> 
> https://git.kernel.dk/cgit/linux-block/commit/?h=perf-wip&id=c754951eb7193258c35a574bd1ccccb7c4946ee4
> 
> so we don't have to dip into the inode itself for the fast path. While
> it's obviously not something being proposed for inclusion right now, is
> there a world in which we can make something like that work?

There's just two places that update i_size for block devices:
set_capacity and bdev_set_nr_sectors.  So you just need to update
bd_nr_sectors there and you're done.