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Date:   Thu, 2 Jun 2022 08:20:08 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Cc:     Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>,
        Ira Weiny <ira.weiny@intel.com>, linux-btrfs@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/3] btrfs: Replace kmap() with kmap_local_page()
Message-ID: <YpjVKAGz+GuI4GB0@infradead.org>
References: <20220531145335.13954-1-fmdefrancesco@gmail.com>
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In-Reply-To: <20220531145335.13954-1-fmdefrancesco@gmail.com>
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Turns out that while this looks good, it actually crashes when
running xfstests.  I think this is due to the fact that kmap sets
the page address, which kmap_local_page does not.

btrfs/150 1s ... [  168.252943] run fstests btrfs/150 at 2022-06-02 15:17:11
[  169.462292] BTRFS info (device vdb): flagging fs with big metadata feature
[  169.463728] BTRFS info (device vdb): disk space caching is enabled
[  169.464953] BTRFS info (device vdb): has skinny extents
[  170.596218] BTRFS: device fsid 37c6bae1-d3e5-47f8-87d5-87cd7240a1b4
devid 1 transid 5 /dev)
[  170.599471] BTRFS: device fsid 37c6bae1-d3e5-47f8-87d5-87cd7240a1b4 devid 2 transid 5 /dev)
[  170.657170] BTRFS info (device vdc): flagging fs with big metadata feature
[  170.659509] BTRFS info (device vdc): use zlib compression, level 3
[  170.661190] BTRFS info (device vdc): disk space caching is enabled
[  170.670706] BTRFS info (device vdc): has skinny extents
[  170.714181] BTRFS info (device vdc): checking UUID tree
[  170.735058] BUG: kernel NULL pointer dereference, address: 0000000000000008
[  170.736478] #PF: supervisor read access in kernel mode
[  170.737457] #PF: error_code(0x0000) - not-present page
[  170.738529] PGD 0 P4D 0 
[  170.739211] Oops: 0000 [#1] PREEMPT SMP PTI
[  170.740101] CPU: 0 PID: 43 Comm: kworker/u4:3 Not tainted 5.18.0-rc7+ #1539
[  170.741478] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
[  170.743246] Workqueue: btrfs-delalloc btrfs_work_helper
[  170.744282] RIP: 0010:zlib_compress_pages+0x128/0x670
[  170.745346] Code: 00 00 00 16 00 00 48 01 e8 31 ed 48 c1 f8 06 48 c1 e0 0c 48 01 f8 49 89 0
[  170.749042] RSP: 0018:ffffc9000038bc70 EFLAGS: 00010286
[  170.750037] RAX: 0000000000000001 RBX: ffffc9000038bdb8 RCX: 0000000000000001
[  170.751351] RDX: 0000000000002000 RSI: ffffffff82f532fb RDI: ffff888000000000
[  170.752695] RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000001
[  170.754106] R10: 0000000000000000 R11: ffff8881039a5b30 R12: ffff888107befb48
[  170.755449] R13: 0000000000000001 R14: 0000000000000000 R15: 0000000000000000
[  170.756922] FS:  0000000000000000(0000) GS:ffff88813bc00000(0000) knlGS:0000000000000000
[  170.758642] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  170.759714] CR2: 0000000000000008 CR3: 000000010ab60000 CR4: 00000000000006f0
[  170.761082] Call Trace:
[  170.761553]  <TASK>
[  170.761968]  ? _raw_spin_unlock+0x24/0x40
[  170.762776]  btrfs_compress_pages+0xda/0x120
[  170.763682]  compress_file_range+0x3b9/0x840
[  170.764570]  async_cow_start+0xd/0x30
[  170.765308]  ? submit_compressed_extents+0x3c0/0x3c0
[  170.766241]  btrfs_work_helper+0xf5/0x3f0
[  170.767009]  ? lock_is_held_type+0xe2/0x140
[  170.767817]  process_one_work+0x239/0x550
[  170.768633]  ? process_one_work+0x550/0x550
[  170.769447]  worker_thread+0x4d/0x3a0
[  170.770210]  ? process_one_work+0x550/0x550
[  170.771019]  kthread+0xe2/0x110
[  170.771623]  ? kthread_complete_and_exit+0x20/0x20
[  170.772697]  ret_from_fork+0x22/0x30
[  170.773454]  </TASK>