Re: [RFC] nvfs: a filesystem for persistent memory

[Mikulas Patocka]

On Tue, 15 Sep 2020, Dan Williams wrote:

> > - when the fsck.nvfs tool mmaps the device /dev/pmem0, the kernel uses
> > buffer cache for the mapping. The buffer cache slows does fsck by a factor
> > of 5 to 10. Could it be possible to change the kernel so that it maps DAX
> > based block devices directly?
> 
> We've been down this path before.
> 
> 5a023cdba50c block: enable dax for raw block devices
> 9f4736fe7ca8 block: revert runtime dax control of the raw block device
> acc93d30d7d4 Revert "block: enable dax for raw block devices"

It says "The functionality is superseded by the new 'Device DAX' 
facility". But the fsck tool can't change a fsdax device into a devdax 
device just for checking. Or can it?

> EXT2/4 metadata buffer management depends on the page cache and we
> eliminated a class of bugs by removing that support. The problems are
> likely tractable, but there was not a straightforward fix visible at
> the time.

Thinking about it - it isn't as easy as it looks...

Suppose that the user mounts an ext2 filesystem and then uses the tune2fs 
tool on the mounted block device. The tune2fs tool reads and writes the 
mounted superblock directly.

So, read/write must be coherent with the buffer cache (otherwise the 
kernel would not see the changes written by tune2fs). And mmap must be 
coherent with read/write.

So, if we want to map the pmem device directly, we could add a new flag 
MAP_DAX. Or we could test if the fd has O_DIRECT flag and map it directly 
in this case. But the default must be to map it coherently in order to not 
break existing programs.

> > - __copy_from_user_inatomic_nocache doesn't flush cache for leading and
> > trailing bytes.
> 
> You want copy_user_flushcache(). See how fs/dax.c arranges for
> dax_copy_from_iter() to route to pmem_copy_from_iter().

Is it something new for the kernel 5.10? I see only __copy_user_flushcache 
that is implemented just for x86 and arm64.

There is __copy_from_user_flushcache implemented for x86, arm64 and power. 
It is used in lib/iov_iter.c under
#ifdef CONFIG_ARCH_HAS_UACCESS_FLUSHCACHE - so should I use this?

Mikulas