linux-kernel - Re: [GIT PULL] x86/mm for 6.4

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Message-ID: <641a9348-a052-6bb5-e6c7-64acb6405328@intel.com>
Date:   Tue, 2 May 2023 08:42:48 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kirill.shutemov@...ux.intel.com,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [GIT PULL] x86/mm for 6.4

On 4/28/23 17:38, Kirill A. Shutemov wrote:
> BTW, I think the static check can be relaxed. Checking size against
> PAGE_SIZE is rather conservative: there's 8 TB (or 4 PB for 5-level
> paging) guard hole at the begging of kernel address space.

Whatever we relax it to, let's make sure we get a note in
Documentation/x86/x86_64/mm.rst.  But that's totally minor and we can
fix it up later.

Have anyone seen any actual code generation difference between:

	return (long)ptr >= 0;

and

        return !((unsigned long)ptr & (1UL<<(BITS_PER_LONG-1)));

?  I'm seeing gcc generate the same code for both the <=PAGE_SIZE side
and the 'sum' side.

It's longer, but I'd rather read the explicit "check bit 63" than the
positive/negative address space thing.  I certainly grok both, but have
to think through the "(long)ptr >= 0" check every time.

I guess it also wouldn't matter as much either if we hid it in a helper
like the attached patch and I didn't have to read it twice. ;)
View attachment "ptr_in_user_half.patch" of type "text/x-patch" (2540 bytes)