linux-kernel - Re: [PATCH] fs/netfs/fscache_io: remove the obsolete "using

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Message-ID: <CAKPOu+8fgsNi3UVfrZQf9WBHwrXq_D=6oauqWJeiOqSeQedgaw@mail.gmail.com>
Date: Mon, 29 Jul 2024 17:35:55 +0200
From: Max Kellermann <max.kellermann@...os.com>
To: Jeff Layton <jlayton@...nel.org>
Cc: dhowells@...hat.com, willy@...radead.org, linux-cachefs@...hat.com, 
	linux-fsdevel@...r.kernel.org, ceph-devel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, stable@...r.kernel.org, xiubli@...hat.com, 
	Ilya Dryomov <idryomov@...il.com>
Subject: Re: [PATCH] fs/netfs/fscache_io: remove the obsolete "using_pgpriv2" flag

On Mon, Jul 29, 2024 at 2:56 PM Jeff Layton <jlayton@...nel.org> wrote:
> Either way, you can add this to both patches:
>
> Reviewed-by: Jeff Layton <jlayton@...nel.org>

Stop the merge :-)

I just found that my patch introduces another lockup; copy_file_range
locks up this way:

 [<0>] folio_wait_private_2+0xd9/0x140
 [<0>] ceph_write_begin+0x56/0x90
 [<0>] generic_perform_write+0xc0/0x210
 [<0>] ceph_write_iter+0x4e2/0x650
 [<0>] iter_file_splice_write+0x30d/0x550
 [<0>] splice_file_range_actor+0x2c/0x40
 [<0>] splice_direct_to_actor+0xee/0x270
 [<0>] splice_file_range+0x80/0xc0
 [<0>] ceph_copy_file_range+0xbb/0x5b0
 [<0>] vfs_copy_file_range+0x33e/0x5d0
 [<0>] __x64_sys_copy_file_range+0xf7/0x200
 [<0>] do_syscall_64+0x64/0x100
 [<0>] entry_SYSCALL_64_after_hwframe+0x76/0x7e

Turns out that there are still private_2 users left in both fs/ceph
and fs/netfs. My patches fix one problem, but cause another problem.
Too bad!

This leaves me confused again: how shall I fix this? Can all
folio_wait_private_2() calls simply be removed?
This looks like some refactoring gone wrong, and some parts don't make
sense (like netfs and ceph claim ownership of the folio_private
pointer). I could try to fix the mess, but I need to know how this is
meant to be. David, can you enlighten me?

Max