Re: [PATCH v3 2/3] powerpc: get hugetlbpage handling more generic

On Tue, 2016-12-06 at 07:34 +0100, Christophe LEROY wrote:

Le 06/12/2016 à 02:18, Scott Wood a écrit :

quoted

On Wed, 2016-09-21 at 10:11 +0200, Christophe Leroy wrote:

quoted

Today there are two implementations of hugetlbpages which are managed
by exclusive #ifdefs:
* FSL_BOOKE: several directory entries points to the same single
hugepage
* BOOK3S: one upper level directory entry points to a table of hugepages

In preparation of implementation of hugepage support on the 8xx, we
need a mix of the two above solutions, because the 8xx needs both cases
depending on the size of pages:
* In 4k page size mode, each PGD entry covers a 4M bytes area. It means
that 2 PGD entries will be necessary to cover an 8M hugepage while a
single PGD entry will cover 8x 512k hugepages.
* In 16 page size mode, each PGD entry covers a 64M bytes area. It means
that 8x 8M hugepages will be covered by one PGD entry and 64x 512k
hugepages will be covers by one PGD entry.

This patch:
* removes #ifdefs in favor of if/else based on the range sizes
* merges the two huge_pte_alloc() functions as they are pretty similar
* merges the two hugetlbpage_init() functions as they are pretty similar

[snip]

quoted

@@ -860,16 +803,34 @@ static int __init hugetlbpage_init(void)

 		 * if we have pdshift and shift value same, we don't
 		 * use pgt cache for hugepd.
 		 */
-		if (pdshift != shift) {
+		if (pdshift > shift) {
 			pgtable_cache_add(pdshift - shift, NULL);
 			if (!PGT_CACHE(pdshift - shift))
 				panic("hugetlbpage_init(): could not
create
"
 				      "pgtable cache for %d bit
pagesize\n", shift);
 		}
+#ifdef CONFIG_PPC_FSL_BOOK3E
+		else if (!hugepte_cache) {

This else never triggers on book3e, because the way this function
calculates
pdshift is wrong for book3e (it uses PyD_SHIFT instead of
HUGEPD_PxD_SHIFT).
 We later get OOMs because huge_pte_alloc() calculates pdshift correctly,
tries to use hugepte_cache, and fails.

Ok, I'll check it again, I was expecting it to still work properly on 
book3e, because after applying patch 3 it works properly on the 8xx.

On 8xx you probably happen to have a page size that yields "pdshift <= shift"
even with the incorrect pdshift calculation, causing hugepte_cache to be
allocated.  The smallest hugepage size on 8xx is 512k compared to 4M on fsl-
book3e.

-Scott