On Mon, Aug 07, 2006 at 05:18:37PM -0400, Shailabh Nagar wrote:
> ...so could we work out how & if configfs' 4K limit on attributes
> can be removed ?
The 4K limit is one of memory. It's straight from the sysfs
code. A page is allocated and pinned while the attribute file is opened.
I don't think we should be arbitrarily allocating large amounts of RAM
here.
Actually, that buffer is perhaps large. Maybe it should be only
1K for simple attributes, though grabbing a page is easy.
So, large things demand a lot of memory, or a lot of thought.
Let's consider something similar to seq_file, but controlled by sysfs.
That is, you create a configfs_large_attribute structure, in which you
specify your seq_show, seq_start, seq_next function pointers. Configfs,
when asked for this object, will do the seq_open() for you, and will
create the file_operations for you. Basically, you have the
functionality of seq_file, but configfs is controlling the inode/dentry
lifetimes.
Now, reading from this object is pretty simple. But what about
writing to it? How big can a single write be -- there is no
seq_write_file yet? Should we allocate that same page buffer if opened
O_WRITE? Try and come up with a seq_write_file?
Also, what does a write do? Append to the file, or replace it?
Almost attributes do a replace, which is a fair thing to do
(open()+write() expects to be starting at 0 unless O_APPEND is
specified). But ckrm looks to want append semantics. If this is
genericized, how does one truncate if "append" is the default? If the
write replaces the existing data, we'd really need a seq_write_file, so
that multiple values can be written across multiple write calls.
These are the sorts of things I'm pondering over this.
> Could you elaborate the uncontrolled set of lifetime semantics part ?
No generic files in configfs. Nothing that isn't created by the
configfs infrastructure. The dirents and inodes are completely
controlled by configfs, and the client module just defines what is
needed.
There will be no kernel-side "sysfs_create(parent, inode)" type
thing.
Joel
--
Life's Little Instruction Book #157
"Take time to smell the roses."
Joel Becker
Principal Software Developer
Oracle
E-mail: [EMAIL PROTECTED]
Phone: (650) 506-8127
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