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From: Daniel Xu <dxu@dxuuu.xyz>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: bpf@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	kernel-team@fb.com, jolsa@kernel.org, hannes@cmpxchg.org,
	yhs@fb.com
Subject: Re: [RFC bpf-next 1/1] bpf: Introduce iter_pagecache
Date: Thu, 8 Apr 2021 13:49:35 -0700
Message-ID: <20210408204935.4itnxm4ekdv7zlrw@dlxu-fedora-R90QNFJV> (raw)
In-Reply-To: <YG8zMV59hSzpCHSn@zeniv-ca.linux.org.uk>

On Thu, Apr 08, 2021 at 04:45:37PM +0000, Al Viro wrote:
> On Wed, Apr 07, 2021 at 02:46:11PM -0700, Daniel Xu wrote:
> 
> > +static void fini_seq_pagecache(void *priv_data)
> > +{
> > +	struct bpf_iter_seq_pagecache_info *info = priv_data;
> > +	struct radix_tree_iter iter;
> > +	struct super_block *sb;
> > +	void **slot;
> > +
> > +	radix_tree_for_each_slot(slot, &info->superblocks, &iter, 0) {
> > +		sb = (struct super_block *)iter.index;
> > +		atomic_dec(&sb->s_active);
> > +		radix_tree_delete(&info->superblocks, iter.index);
> > +	}
> 
> ... and if in the meanwhile all other contributors to ->s_active have
> gone away, that will result in...?

Ah right, sorry. Nobody will clean up the super_block.

> IOW, NAK.  The objects you are playing with have non-trivial lifecycle
> and poking into the guts of data structures without bothering to
> understand it is not a good idea.
> 
> Rule of the thumb: if your code ends up using fields that are otherwise
> handled by a small part of codebase, the odds are that you need to be
> bloody careful.  In particular, ->ns_lock has 3 users - all in
> fs/namespace.c.  ->list/->mnt_list: all users in fs/namespace.c and
> fs/pnode.c.  ->s_active: majority in fs/super.c, with several outliers
> in filesystems and safety of those is not trivial.
> 
> Any time you see that kind of pattern, you are risking to reprise
> a scene from The Modern Times - the one with Charlie taking a trip
> through the guts of machinery.

I'll take a closer look at the lifetime semantics.

Hopefully the overall goal of the patch is ok. Happy to iterate on the
implementation details until it's correct.

Thanks,
Daniel

  reply index

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-07 21:46 [RFC bpf-next 0/1] bpf: Add page cache iterator Daniel Xu
2021-04-07 21:46 ` [RFC bpf-next 1/1] bpf: Introduce iter_pagecache Daniel Xu
2021-04-08  6:14   ` Matthew Wilcox
2021-04-08 19:48     ` Daniel Xu
2021-04-08 21:29       ` Matthew Wilcox
2021-04-08  8:19   ` Christian Brauner
2021-04-08 20:44     ` Daniel Xu
2021-04-08 16:45   ` Al Viro
2021-04-08 20:49     ` Daniel Xu [this message]
2021-04-08 21:04       ` Al Viro
2021-04-08 22:11   ` Dave Chinner
2021-04-08  7:51 ` [RFC bpf-next 0/1] bpf: Add page cache iterator Christian Brauner
2021-04-08 16:08   ` Daniel Xu
2021-04-08 21:33 ` Shakeel Butt
2021-04-08 23:13 ` Darrick J. Wong
2021-04-09  0:24   ` Daniel Xu

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