Messages in this thread | | | Date | Mon, 31 Oct 2005 23:19:21 +0100 | From | Jesper Juhl <> | Subject | Re: [PATCH 2/9] mm: arm ready for split ptlock |
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On 10/26/05, Russell King <rmk+lkml@arm.linux.org.uk> wrote: > On Tue, Oct 25, 2005 at 11:00:09AM -0400, Nicolas Pitre wrote: > > On Tue, 25 Oct 2005, Russell King wrote: > > > > > On Mon, Oct 24, 2005 at 10:45:04PM -0400, Nicolas Pitre wrote: > > > > On Sat, 22 Oct 2005, Russell King wrote: > > > > > Please contact Nicolas Pitre about that - that was my suggestion, > > > > > but ISTR apparantly the overhead is too high. > > > > > > > > Going through a kernel buffer will simply double the overhead. Let's > > > > suppose it should not be a big enough issue to stop the patch from being > > > > merged though (and it looks cleaner that way). However I'd like for the > > > > WARN_ON((unsigned long)frame & 7) to remain as both the kernel and user > > > > buffers should be 64-bit aligned. > > > > > > The WARN_ON is pointless because we guarantee that the stack is always > > > 64-bit aligned on signal handler setup and return. > > > > Sure, but the iWMMXt context is stored after the standard sigcontext > > which also must be 64 bits in size (which might not be always the case > > if things change in the structure or in its padding). > > > > > > I don't see how standard COW could not happen. The only difference with > > > > a true write fault as if we used put_user() is that we bypassed the data > > > > abort vector and the code to get the FAR value. Or am I missing > > > > something? > > > > > > pte_write() just says that the page _may_ be writable. It doesn't say > > > that the MMU is programmed to allow writes. If pte_dirty() doesn't > > > return true, that means that the page is _not_ writable from userspace. > > > > Argh... So only suffice to s/pte_write/pte_dirty/ I'd guess? > > No. If we're emulating a cmpxchg() on a clean BSS page, this code > as it stands today will write to the zero page making it a page which > is mostly zero. Bad news when it's mapped into other processes BSS > pages. > > Changing this for pte_dirty() means that we'll refuse to do a cmpxchg() > on a clean BSS page. The data may compare correctly, but because it > isn't already dirty, you'll fail. > > If we still had it, I'd say you need to use verify_area() to tell the > kernel to pre-COW the pages. However, that got removed a while back. >
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.
-- Jesper Juhl <jesper.juhl@gmail.com> Don't top-post http://www.catb.org/~esr/jargon/html/T/top-post.html Plain text mails only, please http://www.expita.com/nomime.html - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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