lkml.org 
[lkml]   [2015]   [Jul]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v2 3/6] ext4: Use ext4_get_block_write() for DAX
On 07/03/2015 10:07 PM, Theodore Ts'o wrote:
> On Fri, Jul 03, 2015 at 02:48:24PM -0400, Matthew Wilcox wrote:
>>
>> At boot, I "modprobe pmem".
>
> Is there a reason why it's important to build and load pmem as a
> module? If I use CONFIG_BLK_DEV_PMEM=y (which is more convenient
> given how I launch my KVM test appliance), should I expect any
> problems?
>

This (=y) should work fine. We use it a lot. (with KVM even boot an
image with -dax, with a trick)

Note that DAX need not be tested with pmem only, you can always use brd
at any given point without any reboot.

One more trick for xfstest I use:
memmap=2G!4G,2G!6G

And have two pmem0/1 and don't need to bother with any fdisk. Do need
to mkfs every boot though.

BTW: with kvm a reboot with above memmap will give you back the exact
same memory. halt and "virsh start" is a different story.

> I assume that this won't detect any bugs caused by missing CLFLUSH
> instructions, but I assume that when using NVM as a block device, this
> isn't much of an issue, as long as we don't care about torn writes?
> (How using NVM with metdata checksums, or any checksums for that
> matter, seems to be an interesting question --- how do we recover from
> a checksum failure after a power failure?)
>

Currently pmem maps a very-(very) slow ioremap_nocache. So any Kernel
memory access should be pmem persistent. For a real world faster ioremap,
there are few major pieces still missing in the stack to make it
persistent. Note the even today with ioremap_nocache, any application
mmap (like git) is not persistent.

> - Ted

Cheers
Boaz



\
 
 \ /
  Last update: 2015-07-05 15:41    [W:0.034 / U:0.648 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site