linux-ext4 - Re: [PATCH] ext4: drop file_update_time from ext4_dax

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Date:	Thu, 19 Nov 2015 20:39:06 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	"Ted Ts'o" <tytso@....edu>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Jan Kara <jack@...e.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	ext4 hackers <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] ext4: drop file_update_time from ext4_dax_fault

On Thu, Nov 19, 2015 at 7:03 PM, Dave Chinner <david@...morbit.com> wrote:
> On Thu, Nov 19, 2015 at 04:18:13PM -0800, Dan Williams wrote:
>> Neither the filemap_fault() nor the xfs dax fault path is updating time.
>> This call leads to the following WARN() when the block device has been
>> torn down:
>
> I don't think that is right. In xfs_filemap_fault():
>
>
> ....
>         /* DAX can shortcut the normal fault path on write faults! */
>         if ((vmf->flags & FAULT_FLAG_WRITE) && IS_DAX(inode))
>                 return xfs_filemap_page_mkwrite(vma, vmf);
> ....
>
> And xfs_filemap_page_mkwrite() most definitely calls file_update_time():
>
> ....
>         trace_xfs_filemap_page_mkwrite(XFS_I(inode));
>
>         sb_start_pagefault(inode->i_sb);
>         file_update_time(vma->vm_file);
>         xfs_ilock(XFS_I(inode), XFS_MMAPLOCK_SHARED);
> ....
>
> And, finally, in xfs_filemap_pmd_fault():
>
> ....
>         if (flags & FAULT_FLAG_WRITE) {
>                 sb_start_pagefault(inode->i_sb);
>                 file_update_time(vma->vm_file);
>         }
> ....
>
> So we are clearly updating timestamps in XFS on every write fault
> that occurs, whether it be through ->page_mkwrite, ->fault or
> ->pmd_fault. Hence removing those from ext4 can't be the righ tthing
> to do.
>

Ah sorry I missed that.  When I saw that xfs did not trigger the same
warning as ext4 I just assumed it wasn't doing the time update.

>>
>>  WARNING: CPU: 0 PID: 2133 at fs/fs-writeback.c:2065 __mark_inode_dirty+0x261/0x350()
>>  bdi-block not registered
>    ^^^^^^^^^^^^^^^^^^^^^^^^
>>  [..]
>>  Call Trace:
>>   [<ffffffff81459f62>] dump_stack+0x44/0x62
>>   [<ffffffff810a2052>] warn_slowpath_common+0x82/0xc0
>>   [<ffffffff810a20ec>] warn_slowpath_fmt+0x5c/0x80
>>   [<ffffffff812831a1>] __mark_inode_dirty+0x261/0x350
>>   [<ffffffff8126d109>] generic_update_time+0x79/0xd0
>>   [<ffffffff8126d28d>] file_update_time+0xbd/0x110
>>   [<ffffffff812e4bc8>] ext4_dax_fault+0x68/0x110
>>   [<ffffffff811f816e>] __do_fault+0x4e/0xf0
>>   [<ffffffff811fc2a7>] handle_mm_fault+0x5e7/0x1b50
>>   [<ffffffff811fbd11>] ? handle_mm_fault+0x51/0x1b50
>>   [<ffffffff810689c1>] __do_page_fault+0x191/0x3f0
>>   [<ffffffff81068cef>] trace_do_page_fault+0x4f/0x120
>>   [<ffffffff8106314a>] do_async_page_fault+0x1a/0xa0
>>   [<ffffffff81902678>] async_page_fault+0x28/0x30
>
> Doesn't this indicate some problem at the block/bdi level?
> __mark_inode_dirty() should not throw warnings like this regardless
> of where it is called from...
>

I'll look closer at how the xfs path avoids triggering this...
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