Re: [PATCH V4 03/18] powerpc/mm: add _PAGE_HASHPTE similar to 4K hash

Paul Mackerras [off-list ref] writes:

On Tue, Feb 23, 2016 at 10:18:05AM +0530, Aneesh Kumar K.V wrote:

quoted

The difference between 64K and 4K hash fault handling is confusing
with respect to when we set _PAGE_HASHPTE in the linux pte.
I was trying to find out whether we miss a hpte flush in any
scenario because of this. ie, a pte update on a linux pte, for which we
are doing a parallel hash pte insert. After looking at it closer my
understanding is this won't happen because pte update also look at
_PAGE_BUSY and we will wait for hash pte insert to finish before going
ahead with the pte update. But to avoid further confusion keep the
hash fault handler for all the page size similar to  __hash_page_4k.

This partially reverts commit 41743a4e34f0 ("powerpc: Free a PTE bit on ppc64 with 64K pages"

In each of the functions you are modifying below, there is already an
explicit setting of _PAGE_HASHPTE in new_pte.  So I don't think this
is necessary, or if we do this, we can eliminate the separate setting
of _PAGE_HASHPTE later on.

In general I think it's better to leave the setting of _PAGE_HASHPTE
until we know what slot the HPTE is going to go into.  That way we
have less chance of ending up with _PAGE_HASHPTE set but bogus
information in _PAGE_F_GIX and _PAGE_F_SECOND.

Ok. I will do a variant that remove _PAGE_HASHPTE setting from 4k. ie, 

diff --git a/arch/powerpc/mm/hash64_4k.c b/arch/powerpc/mm/hash64_4k.c
index e3e76b929f33..47d1b26effc6 100644
--- a/arch/powerpc/mm/hash64_4k.c
+++ b/arch/powerpc/mm/hash64_4k.c

@@ -44,7 +44,7 @@ int __hash_page_4K(unsigned long ea, unsigned long access, unsigned long vsid,
 		 * a write access. Since this is 4K insert of 64K page size
 		 * also add _PAGE_COMBO
 		 */
-		new_pte = old_pte | _PAGE_BUSY | _PAGE_ACCESSED | _PAGE_HASHPTE;
+		new_pte = old_pte | _PAGE_BUSY | _PAGE_ACCESSED;
 		if (access & _PAGE_RW)
 			new_pte |= _PAGE_DIRTY;
 	} while (old_pte != __cmpxchg_u64((unsigned long *)ptep,