linux-kernel - Re: [PATCH v3 0/3+1] fanotify accounting for fs/splice.c

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Message-ID: <t5az5bvpfqd3rrwla43437r5vplmkujdytixcxgm7sc4hojspg@jcc63stk66hz>
Date:   Tue, 27 Jun 2023 22:34:06 +0200
From:   Ahelenia Ziemiańska 
        <nabijaczleweli@...ijaczleweli.xyz>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jan Kara <jack@...e.cz>,
        Chung-Chiang Cheng <cccheng@...ology.com>, ltp@...ts.linux.it
Subject: Re: [PATCH v3 0/3+1] fanotify accounting for fs/splice.c

On Tue, Jun 27, 2023 at 09:03:17PM +0300, Amir Goldstein wrote:
> On Tue, Jun 27, 2023 at 7:55 PM Ahelenia Ziemiańska
> <nabijaczleweli@...ijaczleweli.xyz> wrote:
> >
> > In 1/3 I've applied if/else if/else tree like you said,
> > and expounded a bit in the message.
> >
> > This is less pretty now, however, since it turns out that
> If my advice turns out to be bad, then please drop it.
The if/else if/else with no goto is better than before;
it was made ugly by the special-casing below.

> > iter_file_splice_write() already marks the out fd as written because it
> > writes to it via vfs_iter_write(), and that sent a double notification.
> >
> > $ git grep -F .splice_write | grep -v iter_file_splice_write
> > drivers/char/mem.c:     .splice_write   = splice_write_null,
> > drivers/char/virtio_console.c:  .splice_write = port_fops_splice_write,
> > fs/fuse/dev.c:  .splice_write   = fuse_dev_splice_write,
> > fs/gfs2/file.c: .splice_write   = gfs2_file_splice_write,
> > fs/gfs2/file.c: .splice_write   = gfs2_file_splice_write,
> > fs/overlayfs/file.c:    .splice_write   = ovl_splice_write,
> > net/socket.c:   .splice_write = generic_splice_sendpage,
> > scripts/coccinelle/api/stream_open.cocci:    .splice_write = splice_write_f,
> >
> > Of these, splice_write_null() doesn't mark out as written
> > (but it's for /dev/null so I think this is expected),
> > and I haven't been able to visually confirm whether
> > port_fops_splice_write() and generic_splice_sendpage() do.
> >
> > All the others delegate to iter_file_splice_write().
> All this is very troubling to me.
> It translates to a mental model that I cannot remember and
> cannot maintain for fixes whose value are still questionable.
> 
> IIUC, the only thing you need to change in do_splice() for
> making your use case work is to add fsnotify_modify()
> for the splice_pipe_to_pipe() case. Right?
No, all splice/tee/vmsplice cases need to generate modify events for the
output fd. Really, all I/O syscalls do, but those are for today.

> So either make the change that you need, or all the changes
> that are simple to follow without trying to make the world
> consistent
Thus I also originally had all the aforementioned generate access/modify
for in/out.

> - these pipe iterators business is really messy.
> I don't know if avoiding a double event (which is likely not visible)
> is worth the complicated code that is hard to understand.
> 
> > In 2/3 I fixed the vmsplice notification placement
> > (access from pipe, modify to pipe).
> >
> > I'm following this up with an LTP patch, where only sendfile_file_to_pipe
> > passes on 6.1.27-1 and all tests pass on v6.4 + this patchset.
> Were these tests able to detect the double event?
> Maybe it's not visible because double consequent events get merged.
That's how I discovered it, yes. They aren't merged because we'd generate
  modify out  <- from the VFS callback
  access in   <- from do_splice
  modify out  <- ibid.

I agree this got very ugly very fast for a weird edge case ‒
maybe I did get a little over-zealous on having a consistent
"one syscall↔one event for each affected file" model.

OTOH: I've found that just using
	if (ret > 0) {
		fsnotify_modify(out);
		fsnotify_access(in);
	}
does get the events merged from
  modify out  <- from the VFS callback
  modify out  <- from do_splice
  access in   <- ibid.
into
  modify out
  access in
which solves all issues
(reliable wake-up regardless of backing file, no spurious wake-ups)
at no cost. I would've done this originally, but I hadn't known
inotify events get merged :v

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