On 12/5/18 8:03 AM, Qu Wenruo wrote:
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.
The general idea is:
1) there is no page cache involved. read() and write() are like direct
i/o writes in concept. The user buffer is written directly (via what is
essentially a specialized memcpy) to the NVDIMM.
2) for mmap, once the mapping is established and mapped, the file system
is not involved. The application writes directly to the memory as it
would a normal mmap, except it's persistent. All that's required to
ensure persistence is a CPU cache flush. The only way the file system
is involved again is if some operation has occurred to reset the WP bit.
If btrfs_page_mkwrite() can't provide enough control, then I have a
crazy idea.
It can't, because it is only invoked on the page fault path and we want
to try to limit those as much as possible.
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.
There's no way to force a page fault for every mmap read/write. Even if
there was, we wouldn't want that. No user would turn that on when they
can just make similar guarantees in their app (which are typically apps
that do this already) and not pay any performance penalty. The idea
with DAX mmap is that the file system manages the namespace, space
allocation, and permissions. Otherwise we stay out of the way.
-Jeff
--
Jeff Mahoney
SUSE Labs
