Re: [PATCH 1/5] fs/stat: Define DAX statx attribute

Ira Weiny
2019-10-22T16:51:32Z

On Tue, Oct 22, 2019 at 02:32:04PM +0300, Boaz Harrosh wrote:
> On 20/10/2019 18:59, ira.weiny@intel.com wrote:
> > From: Ira Weiny <ira.weiny@intel.com>
> >
> > In order for users to determine if a file is currently operating in DAX
> > mode (effective DAX). Define a statx attribute value and set that
> > attribute if the effective DAX flag is set.
> >
> > To go along with this we propose the following addition to the statx man
> > page:
> >
> > STATX_ATTR_DAX
> >
> > DAX (cpu direct access) is a file mode that attempts to minimize
> > software cache effects for both I/O and memory mappings of this
> > file. It requires a capable device, a compatible filesystem
> > block size, and filesystem opt-in. It generally assumes all
> > accesses are via cpu load / store instructions which can
> > minimize overhead for small accesses, but adversely affect cpu
> > utilization for large transfers. File I/O is done directly
> > to/from user-space buffers. While the DAX property tends to
> > result in data being transferred synchronously it does not give
> > the guarantees of synchronous I/O that data and necessary
> > metadata are transferred. Memory mapped I/O may be performed
> > with direct mappings that bypass system memory buffering. Again
> > while memory-mapped I/O tends to result in data being
> > transferred synchronously it does not guarantee synchronous
> > metadata updates. A dax file may optionally support being mapped
> > with the MAP_SYNC flag which does allow cpu store operations to
> > be considered synchronous modulo cpu cache effects.
> >
> > Signed-off-by: Ira Weiny <ira.weiny@intel.com>
> > ---
> > fs/stat.c | 3 +++
> > include/uapi/linux/stat.h | 1 +
> > 2 files changed, 4 insertions(+)
> >
> > diff --git a/fs/stat.c b/fs/stat.c
> > index c38e4c2e1221..59ca360c1ffb 100644
> > --- a/fs/stat.c
> > +++ b/fs/stat.c
> > @@ -77,6 +77,9 @@ int vfs_getattr_nosec(const struct path *path, struct kstat *stat,
> > if (IS_AUTOMOUNT(inode))
> > stat->attributes |= STATX_ATTR_AUTOMOUNT;
> >
> > + if (inode->i_flags & S_DAX)
>
> Is there a reason not to use IS_DAX(inode) ?

No, just forgot there was a macro when this was written. Changed.

Thanks,
Ira

>
> > + stat->attributes |= STATX_ATTR_DAX;
> > +
> > if (inode->i_op->getattr)
> > return inode->i_op->getattr(path, stat, request_mask,
> > query_flags);
> > diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
> > index 7b35e98d3c58..5b0962121ef7 100644
> > --- a/include/uapi/linux/stat.h
> > +++ b/include/uapi/linux/stat.h
> > @@ -169,6 +169,7 @@ struct statx {
> > #define STATX_ATTR_ENCRYPTED 0x00000800 /* [I] File requires key to decrypt in fs */
> >
> > #define STATX_ATTR_AUTOMOUNT 0x00001000 /* Dir: Automount trigger */
> > +#define STATX_ATTR_DAX 0x00002000 /* [I] File is DAX */
> >
> >
> > #endif /* _UAPI_LINUX_STAT_H */
> >
>