Re: [PATCH] fs/9p: Add new options to Documentation

Eric Van Hensbergen
2023-03-28T15:52:10Z

As I work through the documentation rework and to some extent the
testing matrix -- I am reconsidering some choices and wanted to open
up the discussion here.

TLDR; I'm thinking of reworking the cache options before the merge
window to keep things simple while setting up for some of the future
options.

While we have a bunch of new options, in practice I expect users to
probably consolidate around three models: no caching, tight caches,
and expiring caches with fscache being an orthogonal add-on to the
last two.

The ultimate goal is to simplify the options based on expected use models:

- cache=[ none, file, all ] (none is currently default)
- write_policy = [ *writethrough, writeback ] (writethrough would be default)
- cache_validate = [ never, *open, x (seconds) ] (cache_validate
would default to open)
- fscache

So, mapping of existing (deprecated) legacy modes:
- none (obvious) write_policy=writethrough
- *readahead -> cache=file cache_validate_open write_policy=writethrough
- mmap -> cache=file cache_validate=open write_policy=writeback
- loose -> cache=all cache_validate=never write_policy=writeback
- fscache -> cache=all cache_validate=never write_policy=writeback &
fscache enabled

Some things I'm less certain of: cache_validation is probably an
imperfect term as is using 'open' as one of the options, in this case
I'm envisioning 'open' to mean open-to-close coherency for file
caching (cache is only validated on open) and validation on lookup for
dir-cache coherency (using qid.version). Specifying a number here
expires existing caches and requires validation after a certain number
of seconds (is that the right granularity)?

So, I think this is more clear from a documentation standpoint, but
unfortuantely I haven't reduced the test matrix much - in fact I've
probably made it worse. I expect the common cases to basically be:
- cache=none
- new default? (cache=all, write_policy=writeback, cache_validate=open)
- fscache w/(cache=all, write_policy=writeback, cache_validate=5)

Which would give us 3 configurations to test against versus 25
(assuming testing for one time value for cache-validate=x). Important
to remember that this is just cache mode tests, the other mount
options act as multipliers.

Thoughts? Alternatives?

-eric

On Mon, Mar 27, 2023 at 10:38 AM Christian Schoenebeck
<linux_oss@crudebyte.com> wrote:
>
> On Monday, March 27, 2023 5:05:52 AM CEST Eric Van Hensbergen wrote:
> > Need to update the documentation for new mount flags
> > and cache modes.
> >
> > Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>
> > ---
> > Documentation/filesystems/9p.rst | 29 ++++++++++++++++-------------
> > 1 file changed, 16 insertions(+), 13 deletions(-)
> >
> > diff --git a/Documentation/filesystems/9p.rst b/Documentation/filesystems/9p.rst
> > index 0e800b8f73cc..6d257854a02a 100644
> > --- a/Documentation/filesystems/9p.rst
> > +++ b/Documentation/filesystems/9p.rst
> > @@ -78,19 +78,18 @@ Options
> > offering several exported file systems.
> >
> > cache=mode specifies a caching policy. By default, no caches are used.
> > -
> > - none
> > - default no cache policy, metadata and data
> > - alike are synchronous.
> > - loose
> > - no attempts are made at consistency,
> > - intended for exclusive, read-only mounts
> > - fscache
> > - use FS-Cache for a persistent, read-only
> > - cache backend.
> > - mmap
> > - minimal cache that is only used for read-write
> > - mmap. Northing else is cached, like cache=none
> > + Modes are progressive and inclusive. For example, specifying fscache
> > + will use loose caches, writeback, and readahead. Due to their
> > + inclusive nature, only one cache mode can be specified per mount.
>
> I would highly recommend to rather specify below for each option "this option
> implies writeback, readahead ..." etc., as it is not obvious otherwise which
> option would exactly imply what. It is worth those extra few lines IMO to
> avoid confusion.
>
> > +
> > + ========= =============================================
> > + none no cache of file or metadata
> > + readahead readahead caching of files
> > + writeback delayed writeback of files
> > + mmap support mmap operations read/write with cache
> > + loose meta-data and file cache with no coherency
> > + fscache use FS-Cache for a persistent cache backend
> > + ========= =============================================
> >
> > debug=n specifies debug level. The debug level is a bitmask.
> >
> > @@ -137,6 +136,10 @@ Options
> > This can be used to share devices/named pipes/sockets between
> > hosts. This functionality will be expanded in later versions.
> >
> > + directio bypass page cache on all read/write operations
> > +
> > + ignoreqv ignore qid.version==0 as a marker to ignore cache
> > +
> > noxattr do not offer xattr functions on this mount.
> >
> > access there are four access modes.
> >
>
>
>
>