On Tue, Apr 14, 2020 at 12:04:57PM -0700, Dan Williams wrote: > On Tue, Apr 14, 2020 at 9:15 AM Darrick J. Wong <darrick.wong@xxxxxxxxxx> wrote: > > > > On Mon, Apr 13, 2020 at 10:21:26PM -0700, Dan Williams wrote: > > > On Sun, Apr 12, 2020 at 10:41 PM <ira.weiny@xxxxxxxxx> wrote: > > > > > > > > From: Ira Weiny <ira.weiny@xxxxxxxxx> > > > > > > > > Update the Usage section to reflect the new individual dax selection > > > > functionality. > > > > > > > > Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx> > > > > > > > > --- > > > > Changes from V6: > > > > Update to allow setting FS_XFLAG_DAX any time. > > > > Update with list of behaviors from Darrick > > > > https://lore.kernel.org/lkml/20200409165927.GD6741@magnolia/ > > > > > > > > Changes from V5: > > > > Update to reflect the agreed upon semantics > > > > https://lore.kernel.org/lkml/20200405061945.GA94792@xxxxxxxxxxxxxxxxxxxxxxxxx/ > > > > --- > > > > Documentation/filesystems/dax.txt | 166 +++++++++++++++++++++++++++++- > > > > 1 file changed, 163 insertions(+), 3 deletions(-) > > > > > > > > diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt > > > > index 679729442fd2..af14c1b330a9 100644 > > > > --- a/Documentation/filesystems/dax.txt > > > > +++ b/Documentation/filesystems/dax.txt > > > > @@ -17,11 +17,171 @@ For file mappings, the storage device is mapped directly into userspace. > > > > Usage > > > > ----- > > > > > > > > -If you have a block device which supports DAX, you can make a filesystem > > > > +If you have a block device which supports DAX, you can make a file system > > > > on it as usual. The DAX code currently only supports files with a block > > > > size equal to your kernel's PAGE_SIZE, so you may need to specify a block > > > > -size when creating the filesystem. When mounting it, use the "-o dax" > > > > -option on the command line or add 'dax' to the options in /etc/fstab. > > > > +size when creating the file system. > > > > + > > > > +Currently 2 filesystems support DAX, ext4 and xfs. Enabling DAX on them is > > > > +different at this time. > > > > + > > > > +Enabling DAX on ext4 > > > > +-------------------- > > > > + > > > > +When mounting the filesystem, use the "-o dax" option on the command line or > > > > +add 'dax' to the options in /etc/fstab. > > > > + > > > > + > > > > +Enabling DAX on xfs > > > > +------------------- > > > > + > > > > +Summary > > > > +------- > > > > + > > > > + 1. There exists an in-kernel access mode flag S_DAX that is set when > > > > + file accesses go directly to persistent memory, bypassing the page > > > > + cache. > > > > > > I had reserved some quibbling with this wording, but now that this is > > > being proposed as documentation I'll let my quibbling fly. "dax" may > > > imply, but does not require persistent memory nor does it necessarily > > > "bypass page cache". For example on configurations that support dax, > > > but turn off MAP_SYNC (like virtio-pmem), a software flush is > > > required. Instead, if we're going to define "dax" here I'd prefer it > > > be a #include of the man page definition that is careful (IIRC) to > > > only talk about semantics and not backend implementation details. In > > > other words, dax is to page-cache as direct-io is to page cache, > > > effectively not there, but dig a bit deeper and you may find it. > > > > Uh, which manpage? Are you talking about the MAP_SYNC documentation? > > No, I was referring to the proposed wording for STATX_ATTR_DAX. > There's no reason for this description to say anything divergent from > that description. Ok I think the best text would be to simply refer to the STATX_ATTR_DAX man page here. Something like: <quote> 1. There exists an in-kernel access mode flag S_DAX that is set when file accesses is enabled for 'DAX'. Applications must call statx to discover the current S_DAX state (STATX_ATTR_DAX). See the man page for statx for more details. </quote> Ira
