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SubjectRe: [PATCH] fs/btrfs: Fix raid6 qstripe kmap'ing
On Thu, Feb 04, 2021 at 04:26:08PM +0100, David Sterba wrote:
> 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@intel.com wrote:
> > > From: Ira Weiny <ira.weiny@intel.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@fb.com>
> > > To: Josef Bacik <josef@toxicpanda.com>
> > > To: David Sterba <dsterba@suse.com>
> > > Cc: Miao Xie <miaox@cn.fujitsu.com>
> > > Signed-off-by: Ira Weiny <ira.weiny@intel.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.

:-(

I think I may have been off by 1 on the raid6 kmap...

Something like this should fix it...

diff --git a/fs/btrfs/raid56.c b/fs/btrfs/raid56.c
index b8a39dad0f00..dbf52f1a379d 100644
--- a/fs/btrfs/raid56.c
+++ b/fs/btrfs/raid56.c
@@ -2370,7 +2370,7 @@ static noinline void finish_parity_scrub(struct btrfs_raid_bio *rbio,
goto cleanup;
}
SetPageUptodate(q_page);
- pointers[rbio->real_stripes] = kmap(q_page);
+ pointers[rbio->real_stripes - 1] = kmap(q_page);
}

atomic_set(&rbio->error, 0);
Let me roll a new version.

Sorry,
Ira

>
> 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

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