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Date:   Thu, 8 Apr 2021 21:04:30 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Daniel Xu <dxu@dxuuu.xyz>
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,
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Subject: Re: [RFC bpf-next 1/1] bpf: Introduce iter_pagecache
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On Thu, Apr 08, 2021 at 01:49:35PM -0700, Daniel Xu wrote:

> 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.

That depends.  Note that bumping ->s_active means that umount of that
sucker will *NOT* shut it down - that would happen only on the thread
doing the final deactivation.  What's more, having e.g. a USB stick
mounted, doing umount(1), having it complete successfully, pulling the
damn thing out and getting writes lost would make for a nasty surprise
for users.

With your approach it seems to be inevitable.  Holding namespace_sem
through the entire thing would prevent that, but's it's a non-starter
for other reasons (starting with "it's a system-wide lock, so that'd
be highly antisocial").  Are there any limits on what could be done
to the pages, anyway?  Because if it's "anything user wanted to do",
it's *really* not feasible.