linux-kernel - [rfc 0/3] Cleaning up soft-dirty bit usage

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Date:	Thu, 03 Apr 2014 22:48:44 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	linux-kernel@...r.kernel.org
Cc:	gorcunov@...nvz.org, linux-mm@...ck.org
Subject: [rfc 0/3] Cleaning up soft-dirty bit usage

Hi! I've been trying to clean up soft-dirty bit usage. I can't cleanup
"ridiculous macros in pgtable-2level.h" completely because I need to
define _PAGE_FILE,_PAGE_PROTNONE,_PAGE_NUMA bits in sequence manner
like

#define _PAGE_BIT_FILE		(_PAGE_BIT_PRESENT + 1)	/* _PAGE_BIT_RW */
#define _PAGE_BIT_NUMA		(_PAGE_BIT_PRESENT + 2)	/* _PAGE_BIT_USER */
#define _PAGE_BIT_PROTNONE	(_PAGE_BIT_PRESENT + 3)	/* _PAGE_BIT_PWT */

which can't be done right now because numa code needs to save original
pte bits for example in __split_huge_page_map, if I'm not missing something
obvious.

Also if we ever redefine the bits above we will need to update PAT code
which uses _PAGE_GLOBAL + _PAGE_PRESENT to make pte_present return true
or false.

Another weird thing I found is the following sequence:

   mprotect_fixup
    change_protection (passes @prot_numa = 0 which finally ends up in)
      ...
      change_pte_range(..., prot_numa)

			if (!prot_numa) {
				...
			} else {
				... this seems to be dead code branch ...
			}

    is it intentional, and @prot_numa argument is supposed to be passed
    with prot_numa = 1 one day, or it's leftover from old times?

Note I've not yet tested the series building it now, hopefully finish
testing in a couple of hours.

Linus, by saying "define the bits we use when PAGE_PRESENT==0 separately
and explicitly" you meant complete rework of the bits, right? Not simply
group them in once place in a header?

	Cyrill
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