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    SubjectRe: [PATCH 2/9] mm: arm ready for split ptlock
    On Mon, Oct 31, 2005 at 11:34:39PM +0100, Jesper Juhl wrote:
    > On 10/31/05, Russell King <rmk+lkml@arm.linux.org.uk> wrote:
    > > On Mon, Oct 31, 2005 at 11:19:21PM +0100, Jesper Juhl wrote:
    > > > Yes, I removed verify_area() since it was just a wrapper for access_ok().
    > > > If verify_area() was/is needed, then access_ok() should be just fine
    > > > as a replacement as far as I can see.
    > >
    > > Except verify_area() would pre-fault the pages in whereas access_ok()
    > > just verifies that the address is a user page. That's quite important
    > > in this case because in order to fault the page in, we need to use
    > > put_user() to get the permission checking correct.
    > >
    > > However, we can't use put_user() because then the cmpxchg emulation
    > > becomes completely non-atomic.
    > >
    > Colour me stupid, but I don't see how that can be.
    >
    > Looking at verify_area() from 2.6.13 - the arm version (
    > http://sosdg.org/~coywolf/lxr/source/include/asm-arm/uaccess.h?v=2.6.13#L81
    > ) :
    >
    > static inline int __deprecated verify_area(int type, const void __user
    > *addr, unsigned long size)
    > {
    > return access_ok(type, addr, size) ? 0 : -EFAULT;
    > }
    >
    > How will this cause pre-faulting if a call to access_ok() will not?
    > Please enlighten me.

    This is true because we haven't had this requirement up until the
    introduction of this. I am referring to some functionality which
    was present a while back in the kernel - the old version of
    verify_area which, when passed a VERIFY_WRITE argument, would
    prefault the pages... as required for some i386 CPUs.

    --
    Russell King
    Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/
    maintainer of: 2.6 Serial core
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