linux-kernel - Re: [RFC bpf-next 1/1] bpf: Introduce iter

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Date:   Thu, 8 Apr 2021 13:49:35 -0700
From:   Daniel Xu <dxu@...uu.xyz>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     bpf@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, jolsa@...nel.org, hannes@...xchg.org,
        yhs@...com
Subject: Re: [RFC bpf-next 1/1] bpf: Introduce iter_pagecache

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