On Tue, 2015-07-21 at 10:05 +0200, Ingo Molnar wrote:

* Toshi Kani [off-list ref] wrote:

quoted

The PAT bit gets relocated to bit 12 when PUD and PMD mappings are 
used.
This bit 12, however, is not covered by PTE_FLAGS_MASK, which is 
corrently
used for masking pfn and flags for all cases.

Patch 1/4-2/4 make changes necessary for patch 3/4 to use 
P?D_PAGE_MASK.

Patch 3/4 fixes pud/pmd interfaces to handle the PAT bit when PUD and 
PMD
mappings are used.

Patch 3/4 fixes /sys/kernel/debug/kernel_page_tables to show the PAT 
bit
properly.

Note, the PAT bit is first enabled in 4.2-rc1 with WT mappings.

Are patches 1-3 only needed to fix /sys/kernel/debug/kernel_page_tables 
output, or 
are there other things fixed as well? The patches do not tell us any of 
that information ...

Patch 3 (and patch 1-2 needed for patch 3) fixes multiple pud/pmd
interfaces to work properly with _PAGE_PAT_LARGE bit set.  Because pmem is
the only module that can create a range with this bit set with large page
WT maps in 4.2, this issue has not been exposed other than the case in
kernel_page_tables fixed by patch 4.  Since there can be other cases in
future, all patches should go to 4.2 to prevent them to happen.  There is
no issue in 4.1 & older since they cannot set the bit.

Thanks,
-Toshi

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