fixing redundant network opens on Linux file creation

[Steven French]

On Mon, Jan 06, 2003 at 10:14:10AM -0800, Richard Sharpe wrote:
>
> Isn't creat() a legacy call? I have never used it, and use open(...,
> O_CREAT,...) instead.
>
> Isn't this just a cost of using legacy calls? Why complicate things
overly
> for a call that might not be used all that much?

As Jan indicated, it looks like creat(filename,mode) and open(filename,
O_CREAT | O_TRUNC, mode) follow similar paths in the Linux kernel and both
have the potential "redundant" network file open problem.   In addition the
Connectathon "nfs" (posix file API compliance) tests do issue the creat()
call (a lot) so it is hard to avoid whether even if some considered it
legacy baggage.    Although it would make sense intuitively that passing
O_CREAT on the open system call would (only) invoke the vfs open call (with
the O_CREAT) flag - it first seems to call the vfs create call (and then
invoke open later with the O_CREAT flag set) and the obvious idea of simply
having some network filesystems simply not exporting a create vfs entry
point would result in EACCES being returned (rather than the vfs invoking
the filesystem's open routine specifying the O_CREAT flag as one might
expect).   Uggh.

I too am intrigued about the idea of a "lookup intent" change and better
optimizing the namei.c/open.c file creation code path for this common case
but I didn't see a quick, low risk (for 2.5 kernel) change that would help
network filesystems so am toying with "opbatch/lazy close" ideas.

Steve French
Senior Software Engineer
Linux Technology Center - IBM Austin
phone: 512-838-2294
email: sfrench at us.ibm.com