On 12.04.21 10:06, Anshuman Khandual wrote:
+ linuxppc-dev@xxxxxxxxxxxxxxxx + linux-ia64@xxxxxxxxxxxxxxx On 4/12/21 9:18 AM, Anshuman Khandual wrote:pageblock_order must always be less than MAX_ORDER, otherwise it might lead to an warning during boot. A similar problem got fixed on arm64 platform with the commit 79cc2ed5a716 ("arm64/mm: Drop THP conditionality from FORCE_MAX_ZONEORDER"). Assert the above condition before HUGETLB_PAGE_ORDER gets assigned as pageblock_order. This will help detect the problem earlier on platforms where HUGETLB_PAGE_SIZE_VARIABLE is enabled. Cc: David Hildenbrand <david@xxxxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: linux-mm@xxxxxxxxx Cc: linux-kernel@xxxxxxxxxxxxxxx Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx> --- Changes in V2: - Changed WARN_ON() to BUILD_BUG_ON() per David Changes in V1: https://patchwork.kernel.org/project/linux-mm/patch/1617947717-2424-1-git-send-email-anshuman.khandual@xxxxxxx/ mm/page_alloc.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index cfc72873961d..19283bff4bec 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -6875,10 +6875,17 @@ void __init set_pageblock_order(void) if (pageblock_order) return;- if (HPAGE_SHIFT > PAGE_SHIFT)+ if (HPAGE_SHIFT > PAGE_SHIFT) { + /* + * pageblock_order must always be less than + * MAX_ORDER. So does HUGETLB_PAGE_ORDER if + * that is being assigned here. + */ + BUILD_BUG_ON(HUGETLB_PAGE_ORDER >= MAX_ORDER);Unfortunately the build test fails on both the platforms (powerpc and ia64) which subscribe HUGETLB_PAGE_SIZE_VARIABLE and where this check would make sense. I some how overlooked the cross compile build failure that actually detected this problem. But wondering why this assert is not holding true ? and how these platforms do not see the warning during boot (or do they ?) at mm/vmscan.c:1092 like arm64 did. static int __fragmentation_index(unsigned int order, struct contig_page_info *info) { unsigned long requested = 1UL << order; if (WARN_ON_ONCE(order >= MAX_ORDER)) return 0; .... Can pageblock_order really exceed MAX_ORDER - 1 ?
Ehm, for now I was under the impression that such configurations wouldn't exist.
And originally, HUGETLB_PAGE_SIZE_VARIABLE was introduced to handle hugepage sizes that all *smaller* than MAX_ORDER - 1: See d9c234005227 ("Do not depend on MAX_ORDER when grouping pages by mobility")
However, looking into init_cma_reserved_pageblock():
if (pageblock_order >= MAX_ORDER) {
i = pageblock_nr_pages;
...
}
But it's kind of weird, isn't it? Let's assume we have MAX_ORDER - 1
correspond to 4 MiB and pageblock_order correspond to 8 MiB.
Sure, we'd be grouping pages in 8 MiB chunks, however, we cannot even allocate 8 MiB chunks via the buddy. So only alloc_contig_range() could really grab them (IOW: gigantic pages).
Further, we have code like deferred_free_range(), where we end up calling __free_pages_core()->...->__free_one_page() with pageblock_order. Wouldn't we end up setting the buddy order to something > MAX_ORDER -1 on that path?
Having pageblock_order > MAX_ORDER feels wrong and looks shaky. -- Thanks, David / dhildenb
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