linux-kernel - Filebench failure on ramdisk with Ext4-DAX

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Date:	Tue, 7 Jul 2015 12:19:48 -0700
From:	Andiry Xu <andiry@...il.com>
To:	Matthew Wilcox <willy@...ux.intel.com>,
	Linux MM <linux-mm@...ck.org>, linux-kernel@...r.kernel.org
Cc:	Andiry Xu <andiry.xu@...il.com>
Subject: Filebench failure on ramdisk with Ext4-DAX

Hi,

I am running into failures when run filebench on ramdisk(/dev/ram0)
with Ext4-DAX.
The kernel version is 4.0, and I also verified it occurs on 4.2-rc1.

The issue reproduction steps:

// Set ramdisk size to 2GB
# mkfs.ext4 /dev/ram0
# mount -o dax /dev/ram0 /mnt/ramdisk
# filebench
filebench> load fileserver
filebench> set $dir=/mnt/ramdisk
filebench> run 30

And filebench fails in a few seconds like this:

8163: 22.992: Failed to pre-allocate file
/mnt/ramdisk/bigfileset/00000001/00000006/00000001/00000024/00000005/00000002/00000006:
No such file or directory on line 128
 8163: 22.992: Failed to create filesets on line 128

Or like this:

8141: 16.372: Failed to write 51967 bytes on fd 23: Success
 8151: 16.372: Failed to write 136735 bytes on fd 18: Success
 8148: 16.372: Failed to write 123317 bytes on fd 31: Success
 8141: 16.381: filereaderthread-36: flowop wrtfile1-1 failed
 8151: 16.381: filereaderthread-46: flowop wrtfile1-1 failed
 8148: 16.381: filereaderthread-43: flowop wrtfile1-1 failed
 8098: 16.521: Run took 1 seconds...
 8098: 16.521: NO VALID RESULTS! Filebench run terminated prematurely on line 65
 8098: 16.521: Shutting down processes

Sometimes it succeeds, but the chance is low. The failure rate is 80%+.

Note:
The issues does not occur with normal Ext4.
The issues does not occur with Ext4-DAX on pmem driver (from 01org/prd).

The only significant difference between brd.c and pmem.c is that brd.c
uses alloc_page() and pmem.c reserved memory range and uses ioremap()
to get virtual address. I assume that the memcpy
operation(copy_from/to_user) directly between user buffer and page by
alloc_page() does not work correctly somehow. I wonder if this is a
bug? If it is, how to fix it? Thanks.

Thanks,
Andiry
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