Re: [PATCH] ext4: drop file_update_time from ext4_dax_fault

[Dave Chinner <david@fromorbit.com>]

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.

> 
>  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...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com