[RFC] speeding up the stat() family of system calls...

[Linus Torvalds]

Here's both x86 people and filesystem people involved, because this
hacky RFC patch touches both.

NOTE NOTE NOTE! I've modified "cp_new_stat()" in place, in a way that
is x86-64 specific. So the attached patch *only* works on x86-64, and
will very actively break on anything else. That's intentional, because
that way it's more obvious how the code changes, but a real patch
would create a *new* cp_new_stat() for x86-64, and conditionalize the
existing generic "cp_new_stat()" on not already having an
architecture-optimized one.

Basically, under some filesystem loads, "stat()" and friends are the
most common ops (think tree traversal, but also things like git
verifying the index). And our "cp_new_stat()" function (which is the
common interface, ignoring 32-bit legacy stuff) is generic, but
actually pretty disgusting. It copies things to a temporary 'struct
stat' buffer on the kernel stack, and then uses copy_to_user() to copy
it to user space. The double copy is quite noticeable in profiles, and
it generates a big stack frame too.

By doing an architecture-specific cp_new_stat() function, we can
improve on that.

HOWEVER. On x86, doing an efficient field-at-a-time copy also requires
us to use put_user_try() and put_user_catch() in order to not have
tons of clac/stac instructions for the extended permission testing.
And the implementation of that was actually fairly non-optimal, so to
actually get the code I wanted, I had to change how that all worked
too, using "asm_volatile_goto()".

Thus both x86 and FS people on the list.

Comments? This would obviously be a 3.14 issue, I'm not suggesting
we'd do this now. I just want to lay the ground-work..

It's tested in the sense that "it works for me", and profiles look nice, but..

               Linus

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