linux-ext4 - Re: [PATCH v2 0/29] block: Make blkdev_get_by

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Message-ID: <20230825134756.o3wpq6bogndukn53@quack3>
Date:   Fri, 25 Aug 2023 15:47:56 +0200
From:   Jan Kara <jack@...e.cz>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
        linux-block@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
        Alasdair Kergon <agk@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anna Schumaker <anna@...nel.org>, Chao Yu <chao@...nel.org>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Dave Kleikamp <shaggy@...nel.org>,
        David Sterba <dsterba@...e.com>, dm-devel@...hat.com,
        drbd-dev@...ts.linbit.com, Gao Xiang <xiang@...nel.org>,
        Jack Wang <jinpu.wang@...os.com>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        jfs-discussion@...ts.sourceforge.net,
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        Joseph Qi <joseph.qi@...ux.alibaba.com>,
        Kent Overstreet <kent.overstreet@...il.com>,
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        reiserfs-devel@...r.kernel.org,
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        Song Liu <song@...nel.org>,
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        xen-devel@...ts.xenproject.org, Jens Axboe <axboe@...nel.dk>,
        Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v2 0/29] block: Make blkdev_get_by_*() return handle

On Fri 25-08-23 02:58:43, Al Viro wrote:
> On Fri, Aug 11, 2023 at 01:04:31PM +0200, Jan Kara wrote:
> > Hello,
> > 
> > this is a v2 of the patch series which implements the idea of blkdev_get_by_*()
> > calls returning bdev_handle which is then passed to blkdev_put() [1]. This
> > makes the get and put calls for bdevs more obviously matching and allows us to
> > propagate context from get to put without having to modify all the users
> > (again!).  In particular I need to propagate used open flags to blkdev_put() to
> > be able count writeable opens and add support for blocking writes to mounted
> > block devices. I'll send that series separately.
> > 
> > The series is based on Christian's vfs tree as of yesterday as there is quite
> > some overlap. Patches have passed some reasonable testing - I've tested block
> > changes, md, dm, bcache, xfs, btrfs, ext4, swap. This obviously doesn't cover
> > everything so I'd like to ask respective maintainers to review / test their
> > changes. Thanks! I've pushed out the full branch to:
> > 
> > git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs.git bdev_handle
> > 
> > to ease review / testing.
> 
> Hmm...  Completely Insane Idea(tm): how about turning that thing inside out and
> having your bdev_open_by... return an actual opened struct file?
> 
> After all, we do that for sockets and pipes just fine and that's a whole lot
> hotter area.
> 
> Suppose we leave blkdev_open()/blkdev_release() as-is.  No need to mess with
> what we have for normal opened files for block devices.  And have block_open_by_dev()
> that would find bdev, etc., same yours does and shove it into anon file.
> 
> Paired with plain fput() - no need to bother with new primitives for closing.
> With a helper returning I_BDEV(bdev_file_inode(file)) to get from those to bdev.
> 
> NOTE: I'm not suggesting replacing ->s_bdev with struct file * if we do that -
> we want that value cached, obviously.  Just store both...
> 
> Not saying it's a good idea, but... might be interesting to look into.
> Comments?

I can see the appeal of not having to introduce the new bdev_handle type
and just using struct file which unifies in-kernel and userspace block
device opens. But I can see downsides too - the last fput() happening from
task work makes me a bit nervous whether it will not break something
somewhere with exclusive bdev opens. Getting from struct file to bdev is
somewhat harder but I guess a helper like F_BDEV() would solve that just
fine.

So besides my last fput() worry about I think this could work and would be
probably a bit nicer than what I have. But before going and redoing the whole
series let me gather some more feedback so that we don't go back and forth.
Christoph, Christian, Jens, any opinion?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR