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

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Date:   Wed, 7 Dec 2016 07:59:07 +0100
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Scott Wood <oss@...error.net>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v3 2/3] powerpc: get hugetlbpage handling more generic



Le 07/12/2016 à 02:06, Scott Wood a écrit :
> On Tue, 2016-12-06 at 07:34 +0100, Christophe LEROY wrote:
>>
>> Le 06/12/2016 à 02:18, Scott Wood a écrit :
>>>
>>> On Wed, 2016-09-21 at 10:11 +0200, Christophe Leroy wrote:
>>>>
>>>> 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]
>>>>
>>>> @@ -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.
>

Indeed it works because on 8xx, PUD_SHIFT == PMD_SHIFT == PGDIR_SHIFT

Christophe