linux-kernel - Re: [PATCH] fs/btrfs: Fix raid6 qstripe kmap'ing

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Date:   Thu, 4 Feb 2021 16:26:08 +0100
From:   David Sterba <dsterba@...e.cz>
To:     ira.weiny@...el.com, clm@...com, josef@...icpanda.com,
        dsterba@...e.com, Miao Xie <miaox@...fujitsu.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs/btrfs: Fix raid6 qstripe kmap'ing

On Wed, Feb 03, 2021 at 04:56:48PM +0100, David Sterba wrote:
> On Wed, Jan 27, 2021 at 10:15:03PM -0800, ira.weiny@...el.com wrote:
> > From: Ira Weiny <ira.weiny@...el.com>
> > 
> > When a qstripe is required an extra page is allocated and mapped.  There
> > were 3 problems.
> > 
> > 1) There is no reason to map the qstripe page more than 1 time if the
> >    number of bits set in rbio->dbitmap is greater than one.
> > 2) There is no reason to map the parity page and unmap it each time
> >    through the loop.
> > 3) There is no corresponding call of kunmap() for the qstripe page.
> > 
> > The page memory can continue to be reused with a single mapping on each
> > iteration by raid6_call.gen_syndrome() without remapping.  So map the
> > page for the duration of the loop.
> > 
> > Similarly, improve the algorithm by mapping the parity page just 1 time.
> > 
> > Fixes: 5a6ac9eacb49 ("Btrfs, raid56: support parity scrub on raid56")
> > To: Chris Mason <clm@...com>
> > To: Josef Bacik <josef@...icpanda.com>
> > To: David Sterba <dsterba@...e.com>
> > Cc: Miao Xie <miaox@...fujitsu.com>
> > Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> > 
> > ---
> > This was found while replacing kmap() with kmap_local_page().  After
> > this patch unwinding all the mappings becomes pretty straight forward.
> > 
> > I'm not exactly sure I've worded this commit message intelligently.
> > Please forgive me if there is a better way to word it.
> 
> Changelog is good, thanks. I've added stable tags as the missing unmap
> is a potential problem.

There are lots of tests faling, stack traces like below. I haven't seen
anything obvious in the patch so that needs a closer look and for the
time being I can't add the patch to for-next.

 BUG: kernel NULL pointer dereference, address:0000000000000000
 #PF: supervisor write access in kernel mode
 #PF: error_code(0x0002) - not-present page
 PGD 0 P4D 0
 Oops: 0002 [#1] PREEMPT SMP
 CPU: 2 PID: 17173 Comm: kworker/u8:5 Not tainted5.11.0-rc6-default+ #1422
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),BIOS rel-1.12.0-59-gc9ba527-rebuilt.opensuse.org 04/01/2014
 Workqueue: btrfs-rmw btrfs_work_helper [btrfs]
 RIP: 0010:raid6_avx22_gen_syndrome+0x103/0x140 [raid6_pq]
 RSP: 0018:ffffa090042cfcf8 EFLAGS: 00010246
 RAX: ffff9e98e1848e80 RBX: ffff9e98d5849000 RCX:0000000000000020
 RDX: ffff9e98e32be000 RSI: 0000000000000000 RDI:ffff9e98e1848e80
 RBP: 0000000000000000 R08: 0000000000000000 R09:0000000000000001
 R10: ffff9e98e1848e90 R11: ffff9e98e1848e98 R12:0000000000001000
 R13: ffff9e98e1848e88 R14: 0000000000000005 R15:0000000000000002
 FS:  0000000000000000(0000) GS:ffff9e993da00000(0000)knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000000 CR3: 0000000023143003 CR4:0000000000170ea0
 Call Trace:
  finish_parity_scrub+0x47b/0x7a0 [btrfs]
  raid56_parity_scrub_stripe+0x24e/0x260 [btrfs]
  btrfs_work_helper+0xd5/0x1d0 [btrfs]
  process_one_work+0x262/0x5f0
  worker_thread+0x4e/0x300
  ? process_one_work+0x5f0/0x5f0
  kthread+0x151/0x170
  ? __kthread_bind_mask+0x60/0x60