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Re: [PATCH v6 4/7] dax: add support for fsync/msync

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  Re: [PATCH v6 4/7] dax: add support for fsync/msync Dan Williams <dan.j.williams@intel.com> - 2016-01-03 19:20 +0100
    Re: [PATCH v6 4/7] dax: add support for fsync/msync Jan Kara <jack@suse.cz> - 2016-01-05 12:20 +0100
      Re: [PATCH v6 4/7] dax: add support for fsync/msync Ross Zwisler <ross.zwisler@linux.intel.com> - 2016-01-05 17:00 +0100
    Re: [PATCH v6 4/7] dax: add support for fsync/msync Ross Zwisler <ross.zwisler@linux.intel.com> - 2016-01-06 19:20 +0100

#1300356 — Re: [PATCH v6 4/7] dax: add support for fsync/msync

On Wed, Dec 23, 2015 at 11:39 AM, Ross Zwisler
<ross.zwisler@linux.intel.com> wrote:
> To properly handle fsync/msync in an efficient way DAX needs to track dirty
> pages so it is able to flush them durably to media on demand.
>
> The tracking of dirty pages is done via the radix tree in struct
> address_space.  This radix tree is already used by the page writeback
> infrastructure for tracking dirty pages associated with an open file, and
> it already has support for exceptional (non struct page*) entries.  We
> build upon these features to add exceptional entries to the radix tree for
> DAX dirty PMD or PTE pages at fault time.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>

I'm hitting the following report with the ndctl dax test [1] on
next-20151231.  I bisected it to
 commit 3cb108f941de "dax-add-support-for-fsync-sync-v6".  I'll take a
closer look tomorrow, but in case someone can beat me to it, here's
the back-trace:

------------[ cut here ]------------
kernel BUG at fs/inode.c:497!
[..]
CPU: 1 PID: 3001 Comm: umount Tainted: G           O    4.4.0-rc7+ #2412
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
task: ffff8800da2a5a00 ti: ffff880307794000 task.ti: ffff880307794000
RIP: 0010:[<ffffffff81280171>]  [<ffffffff81280171>] clear_inode+0x71/0x80
RSP: 0018:ffff880307797d50  EFLAGS: 00010002
RAX: ffff8800da2a5a00 RBX: ffff8800ca2e7328 RCX: ffff8800da2a5a28
RDX: 0000000000000001 RSI: 0000000000000005 RDI: ffff8800ca2e7530
RBP: ffff880307797d60 R08: ffffffff82900ae0 R09: 0000000000000000
R10: ffff8800ca2e7548 R11: 0000000000000000 R12: ffff8800ca2e7530
R13: ffff8800ca2e7328 R14: ffff8800da2e88d0 R15: ffff8800da2e88d0
FS:  00007f2b22f4a880(0000) GS:ffff88031fc40000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00005648abd933e8 CR3: 000000007f3fc000 CR4: 00000000000006e0
Stack:
ffff8800ca2e7328 ffff8800ca2e7000 ffff880307797d88 ffffffffa01c18af
ffff8800ca2e7328 ffff8800ca2e74d0 ffffffffa01ec740 ffff880307797db0
ffffffff81281038 ffff8800ca2e74c0 ffff880307797e00 ffff8800ca2e7328
Call Trace:
[<ffffffffa01c18af>] xfs_fs_evict_inode+0x5f/0x110 [xfs]
[<ffffffff81281038>] evict+0xb8/0x180
[<ffffffff8128113b>] dispose_list+0x3b/0x50
[<ffffffff81282014>] evict_inodes+0x144/0x170
[<ffffffff8126447f>] generic_shutdown_super+0x3f/0xf0
[<ffffffff81264837>] kill_block_super+0x27/0x70
[<ffffffff81264a53>] deactivate_locked_super+0x43/0x70
[<ffffffff81264e9c>] deactivate_super+0x5c/0x60
[<ffffffff81285aff>] cleanup_mnt+0x3f/0x90
[<ffffffff81285b92>] __cleanup_mnt+0x12/0x20
[<ffffffff810c4f26>] task_work_run+0x76/0x90
[<ffffffff81003e3a>] syscall_return_slowpath+0x20a/0x280
[<ffffffff8192671a>] int_ret_from_sys_call+0x25/0x9f
Code: 48 8d 93 30 03 00 00 48 39 c2 75 23 48 8b 83 d0 00 00 00 a8 20
74 1a a8 40 75 18 48 c7 8
3 d0 00 00 00 60 00 00 00 5b 41 5c 5d c3 <0f> 0b 0f 0b 0f 0b 0f 0b 0f
0b 0f 1f 44 00 00 0f 1f
44 00 00 55
RIP  [<ffffffff81280171>] clear_inode+0x71/0x80
RSP <ffff880307797d50>
---[ end trace 3b1d8898a94a4fc1 ]---

[1]: git://git@github.com:pmem/ndctl.git pending
make TESTS="test/dax.sh" check
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#1301455

On Sun 03-01-16 10:13:06, Dan Williams wrote:
> On Wed, Dec 23, 2015 at 11:39 AM, Ross Zwisler
> <ross.zwisler@linux.intel.com> wrote:
> > To properly handle fsync/msync in an efficient way DAX needs to track dirty
> > pages so it is able to flush them durably to media on demand.
> >
> > The tracking of dirty pages is done via the radix tree in struct
> > address_space.  This radix tree is already used by the page writeback
> > infrastructure for tracking dirty pages associated with an open file, and
> > it already has support for exceptional (non struct page*) entries.  We
> > build upon these features to add exceptional entries to the radix tree for
> > DAX dirty PMD or PTE pages at fault time.
> >
> > Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> 
> I'm hitting the following report with the ndctl dax test [1] on
> next-20151231.  I bisected it to
>  commit 3cb108f941de "dax-add-support-for-fsync-sync-v6".  I'll take a
> closer look tomorrow, but in case someone can beat me to it, here's
> the back-trace:
> 
> ------------[ cut here ]------------
> kernel BUG at fs/inode.c:497!

I suppose this is the check that mapping->nr_exceptional is zero, isn't it?
Hum, I don't see how that could happen given we call
truncate_inode_pages_final() just before the clear_inode() call which
removes all the exceptional entries from the radix tree.  And there's not
much room for a race during umount... Does the radix tree really contain
any entry or is it an accounting bug?

								Honza

> [..]
> CPU: 1 PID: 3001 Comm: umount Tainted: G           O    4.4.0-rc7+ #2412
> Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> task: ffff8800da2a5a00 ti: ffff880307794000 task.ti: ffff880307794000
> RIP: 0010:[<ffffffff81280171>]  [<ffffffff81280171>] clear_inode+0x71/0x80
> RSP: 0018:ffff880307797d50  EFLAGS: 00010002
> RAX: ffff8800da2a5a00 RBX: ffff8800ca2e7328 RCX: ffff8800da2a5a28
> RDX: 0000000000000001 RSI: 0000000000000005 RDI: ffff8800ca2e7530
> RBP: ffff880307797d60 R08: ffffffff82900ae0 R09: 0000000000000000
> R10: ffff8800ca2e7548 R11: 0000000000000000 R12: ffff8800ca2e7530
> R13: ffff8800ca2e7328 R14: ffff8800da2e88d0 R15: ffff8800da2e88d0
> FS:  00007f2b22f4a880(0000) GS:ffff88031fc40000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00005648abd933e8 CR3: 000000007f3fc000 CR4: 00000000000006e0
> Stack:
> ffff8800ca2e7328 ffff8800ca2e7000 ffff880307797d88 ffffffffa01c18af
> ffff8800ca2e7328 ffff8800ca2e74d0 ffffffffa01ec740 ffff880307797db0
> ffffffff81281038 ffff8800ca2e74c0 ffff880307797e00 ffff8800ca2e7328
> Call Trace:
> [<ffffffffa01c18af>] xfs_fs_evict_inode+0x5f/0x110 [xfs]
> [<ffffffff81281038>] evict+0xb8/0x180
> [<ffffffff8128113b>] dispose_list+0x3b/0x50
> [<ffffffff81282014>] evict_inodes+0x144/0x170
> [<ffffffff8126447f>] generic_shutdown_super+0x3f/0xf0
> [<ffffffff81264837>] kill_block_super+0x27/0x70
> [<ffffffff81264a53>] deactivate_locked_super+0x43/0x70
> [<ffffffff81264e9c>] deactivate_super+0x5c/0x60
> [<ffffffff81285aff>] cleanup_mnt+0x3f/0x90
> [<ffffffff81285b92>] __cleanup_mnt+0x12/0x20
> [<ffffffff810c4f26>] task_work_run+0x76/0x90
> [<ffffffff81003e3a>] syscall_return_slowpath+0x20a/0x280
> [<ffffffff8192671a>] int_ret_from_sys_call+0x25/0x9f
> Code: 48 8d 93 30 03 00 00 48 39 c2 75 23 48 8b 83 d0 00 00 00 a8 20
> 74 1a a8 40 75 18 48 c7 8
> 3 d0 00 00 00 60 00 00 00 5b 41 5c 5d c3 <0f> 0b 0f 0b 0f 0b 0f 0b 0f
> 0b 0f 1f 44 00 00 0f 1f
> 44 00 00 55
> RIP  [<ffffffff81280171>] clear_inode+0x71/0x80
> RSP <ffff880307797d50>
> ---[ end trace 3b1d8898a94a4fc1 ]---
> 
> [1]: git://git@github.com:pmem/ndctl.git pending
> make TESTS="test/dax.sh" check
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR
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#1301644

On Tue, Jan 05, 2016 at 12:13:46PM +0100, Jan Kara wrote:
> On Sun 03-01-16 10:13:06, Dan Williams wrote:
> > On Wed, Dec 23, 2015 at 11:39 AM, Ross Zwisler
> > <ross.zwisler@linux.intel.com> wrote:
> > > To properly handle fsync/msync in an efficient way DAX needs to track dirty
> > > pages so it is able to flush them durably to media on demand.
> > >
> > > The tracking of dirty pages is done via the radix tree in struct
> > > address_space.  This radix tree is already used by the page writeback
> > > infrastructure for tracking dirty pages associated with an open file, and
> > > it already has support for exceptional (non struct page*) entries.  We
> > > build upon these features to add exceptional entries to the radix tree for
> > > DAX dirty PMD or PTE pages at fault time.
> > >
> > > Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> > 
> > I'm hitting the following report with the ndctl dax test [1] on
> > next-20151231.  I bisected it to
> >  commit 3cb108f941de "dax-add-support-for-fsync-sync-v6".  I'll take a
> > closer look tomorrow, but in case someone can beat me to it, here's
> > the back-trace:
> > 
> > ------------[ cut here ]------------
> > kernel BUG at fs/inode.c:497!
> 
> I suppose this is the check that mapping->nr_exceptional is zero, isn't it?
> Hum, I don't see how that could happen given we call
> truncate_inode_pages_final() just before the clear_inode() call which
> removes all the exceptional entries from the radix tree.  And there's not
> much room for a race during umount... Does the radix tree really contain
> any entry or is it an accounting bug?
> 
> 								Honza

I think this is a bug with the existing way that we handle PMD faults.  The
issue is that the PMD path doesn't properly remove radix tree entries for zero
pages covering holes.  The PMD path calls unmap_mapping_range() to unmap the
range out of the struct address_space, but it is missing a call to
truncate_inode_pages_range() or similar to clear out those entries in the
radix tree.  Up until now we didn't notice, we just had an orphaned entry in
the radix tree, but with my code we then find the page entry in the radix
tree when handling a PMD fault, we remove it and add in a PMD entry.  This
causes us to be off on both our mapping->nrpages and mapping->nrexceptional
counts.	

In the PTE path we properly remove the pages from the radix tree when
upgrading from a hole to a real DAX entry via the delete_from_page_cache()
call, which eventually calls page_cache_tree_delete().

I'm working on a fix now (and making sure all the above is correct).

- Ross
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#1302951

On Sun, Jan 03, 2016 at 10:13:06AM -0800, Dan Williams wrote:
> On Wed, Dec 23, 2015 at 11:39 AM, Ross Zwisler
> <ross.zwisler@linux.intel.com> wrote:
> > To properly handle fsync/msync in an efficient way DAX needs to track dirty
> > pages so it is able to flush them durably to media on demand.
> >
> > The tracking of dirty pages is done via the radix tree in struct
> > address_space.  This radix tree is already used by the page writeback
> > infrastructure for tracking dirty pages associated with an open file, and
> > it already has support for exceptional (non struct page*) entries.  We
> > build upon these features to add exceptional entries to the radix tree for
> > DAX dirty PMD or PTE pages at fault time.
> >
> > Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> 
> I'm hitting the following report with the ndctl dax test [1] on
> next-20151231.  I bisected it to
>  commit 3cb108f941de "dax-add-support-for-fsync-sync-v6".  I'll take a
> closer look tomorrow, but in case someone can beat me to it, here's
> the back-trace:
> 
> ------------[ cut here ]------------
> kernel BUG at fs/inode.c:497!
> [..]
> CPU: 1 PID: 3001 Comm: umount Tainted: G           O    4.4.0-rc7+ #2412
> Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> task: ffff8800da2a5a00 ti: ffff880307794000 task.ti: ffff880307794000
> RIP: 0010:[<ffffffff81280171>]  [<ffffffff81280171>] clear_inode+0x71/0x80
> RSP: 0018:ffff880307797d50  EFLAGS: 00010002
> RAX: ffff8800da2a5a00 RBX: ffff8800ca2e7328 RCX: ffff8800da2a5a28
> RDX: 0000000000000001 RSI: 0000000000000005 RDI: ffff8800ca2e7530
> RBP: ffff880307797d60 R08: ffffffff82900ae0 R09: 0000000000000000
> R10: ffff8800ca2e7548 R11: 0000000000000000 R12: ffff8800ca2e7530
> R13: ffff8800ca2e7328 R14: ffff8800da2e88d0 R15: ffff8800da2e88d0
> FS:  00007f2b22f4a880(0000) GS:ffff88031fc40000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00005648abd933e8 CR3: 000000007f3fc000 CR4: 00000000000006e0
> Stack:
> ffff8800ca2e7328 ffff8800ca2e7000 ffff880307797d88 ffffffffa01c18af
> ffff8800ca2e7328 ffff8800ca2e74d0 ffffffffa01ec740 ffff880307797db0
> ffffffff81281038 ffff8800ca2e74c0 ffff880307797e00 ffff8800ca2e7328
> Call Trace:
> [<ffffffffa01c18af>] xfs_fs_evict_inode+0x5f/0x110 [xfs]
> [<ffffffff81281038>] evict+0xb8/0x180
> [<ffffffff8128113b>] dispose_list+0x3b/0x50
> [<ffffffff81282014>] evict_inodes+0x144/0x170
> [<ffffffff8126447f>] generic_shutdown_super+0x3f/0xf0
> [<ffffffff81264837>] kill_block_super+0x27/0x70
> [<ffffffff81264a53>] deactivate_locked_super+0x43/0x70
> [<ffffffff81264e9c>] deactivate_super+0x5c/0x60
> [<ffffffff81285aff>] cleanup_mnt+0x3f/0x90
> [<ffffffff81285b92>] __cleanup_mnt+0x12/0x20
> [<ffffffff810c4f26>] task_work_run+0x76/0x90
> [<ffffffff81003e3a>] syscall_return_slowpath+0x20a/0x280
> [<ffffffff8192671a>] int_ret_from_sys_call+0x25/0x9f
> Code: 48 8d 93 30 03 00 00 48 39 c2 75 23 48 8b 83 d0 00 00 00 a8 20
> 74 1a a8 40 75 18 48 c7 8
> 3 d0 00 00 00 60 00 00 00 5b 41 5c 5d c3 <0f> 0b 0f 0b 0f 0b 0f 0b 0f
> 0b 0f 1f 44 00 00 0f 1f
> 44 00 00 55
> RIP  [<ffffffff81280171>] clear_inode+0x71/0x80
> RSP <ffff880307797d50>
> ---[ end trace 3b1d8898a94a4fc1 ]---
> 
> [1]: git://git@github.com:pmem/ndctl.git pending
> make TESTS="test/dax.sh" check

This issue is fixed with patch 2 of v7:

https://lists.01.org/pipermail/linux-nvdimm/2016-January/003888.html

Thanks for the report!
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