Messages in this thread | | | Date | Sun, 21 Jun 2009 10:37:01 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [GIT PULL] core kernel fixes |
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On Sun, 21 Jun 2009, Thomas Gleixner wrote: > > Hmm. The main reason why we switched to get_user_pages_fast() in the > futex code is to avoid mmap_sem contention which was observed as a > real big performance problem especially with those horrible JavaVM > applications.
Not relevant.
get_user_pages_fast() takes the mmap_sem for the case where it needs to fault things in too.
So assuming the _only_ reason this thing is called is because we failed earlier when doing the futex_atomic_op_inuser(), then you're basically guaranteed that the "fast" case of get_user_pages_fast() is never actually taken, since we already know that the page tables aren't amenable to an atomic access.
And as far as I can tell, that is indeed the only case where you use that 'get_user_writeable()' thing. You've had futex_atomic_op_inuser() fail, and need to repeat. No?
> As a fallout of this we got rid of the private find_vma / > handle_mm_fault magic (as above) in the futex code which mm folks > frowned upon for quite a while. Unfortunately we got it wrong :(
Sure. But "get_user_pages_fast()" really is the wrong thing. You're not at all interested in the user pages. You're interested in making sure that the page is atomically writable, and nothing else. Right?
Which is why I said that "lock ; addl $0,(mem)" would be a _single_ instruction, and do everything that your "get_user_pages_fast()" hack would do. If the fault is unlikely, that would be a better operation. I just don't think the fault is unlikely, I suspect it happens every time.
Linus
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