Re: [PATCH 00/10] btrfs: Support for DAX devices

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2018/12/5 下午8:28, Goldwyn Rodrigues wrote:
> This is a support for DAX in btrfs. I understand there have been
> previous attempts at it. However, I wanted to make sure copy-on-write
> (COW) works on dax as well.
> 
> Before I present this to the FS folks I wanted to run this through the
> btrfs. Even though I wish, I cannot get it correct the first time
> around :/.. Here are some questions for which I need suggestions:
> 
> Questions:
> 1. I have been unable to do checksumming for DAX devices. While
> checksumming can be done for reads and writes, it is a problem when mmap
> is involved because btrfs kernel module does not get back control after
> an mmap() writes. Any ideas are appreciated, or we would have to set
> nodatasum when dax is enabled.

I'm not familar with DAX, so it's completely possible I'm talking like
an idiot.

If btrfs_page_mkwrite() can't provide enough control, then I have a
crazy idea.

Forcing page fault for every mmap() read/write (completely disable page
cache like DIO).
So that we could get some control since we're informed to read the page
and do some hacks there.

Thanks,
Qu
> 
> 2. Currently, a user can continue writing on "old" extents of an mmaped file
> after a snapshot has been created. How can we enforce writes to be directed
> to new extents after snapshots have been created? Do we keep a list of
> all mmap()s, and re-mmap them after a snapshot?
> 
> Tested by creating a pmem device in RAM with "memmap=2G!4G" kernel
> command line parameter.
> 
> 
> [PATCH 01/10] btrfs: create a mount option for dax
> [PATCH 02/10] btrfs: basic dax read
> [PATCH 03/10] btrfs: dax: read zeros from holes
> [PATCH 04/10] Rename __endio_write_update_ordered() to
> [PATCH 05/10] btrfs: Carve out btrfs_get_extent_map_write() out of
> [PATCH 06/10] btrfs: dax write support
> [PATCH 07/10] dax: export functions for use with btrfs
> [PATCH 08/10] btrfs: dax add read mmap path
> [PATCH 09/10] btrfs: dax support for cow_page/mmap_private and shared
> [PATCH 10/10] btrfs: dax mmap write
> 
>  fs/btrfs/Makefile   |    1 
>  fs/btrfs/ctree.h    |   17 ++
>  fs/btrfs/dax.c      |  303 ++++++++++++++++++++++++++++++++++++++++++++++++++--
>  fs/btrfs/file.c     |   29 ++++
>  fs/btrfs/inode.c    |   54 +++++----
>  fs/btrfs/ioctl.c    |    5 
>  fs/btrfs/super.c    |   15 ++
>  fs/dax.c            |   35 ++++--
>  include/linux/dax.h |   16 ++
>  9 files changed, 430 insertions(+), 45 deletions(-)
> 
> 


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