On 11/07/2016 03:31 PM, Naoya Horiguchi wrote:

pmd_present() checks _PAGE_PSE along with _PAGE_PRESENT to avoid false negative
return when it races with thp spilt (during which _PAGE_PRESENT is temporary
cleared.) I don't think that dropping _PAGE_PSE check in pmd_present() works
well because it can hurt optimization of tlb handling in thp split.
In the current kernel, bit 6 is not used in non-present format because nonlinear
file mapping is obsolete, so let's move _PAGE_SWP_SOFT_DIRTY to that bit.
Bit 7 is used as reserved (always clear), so please don't use it for other
purpose.

...

 #ifdef CONFIG_MEM_SOFT_DIRTY
-#define _PAGE_SWP_SOFT_DIRTY	_PAGE_PSE
+#define _PAGE_SWP_SOFT_DIRTY	_PAGE_DIRTY
 #else
 #define _PAGE_SWP_SOFT_DIRTY	(_AT(pteval_t, 0))
 #endif

I'm not sure this works.  Take a look at commit 00839ee3b29 and the
erratum it works around.  I _think_ this means that a system affected by
the erratum might see an erroneous _PAGE_SWP_SOFT_DIRTY/_PAGE_DIRTY get
set in swap ptes.

There are much worse things that can happen, but I don't think bits 5
(Accessed) and 6 (Dirty) are good choices since they're affected by the
erratum.

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