linux-kernel - Re: [PATCH v6 4/9] mm: thp: Introduce anon_orders and anon_always

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Date:   Fri, 29 Sep 2023 15:55:30 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Ryan Roberts <ryan.roberts@....com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Yu Zhao <yuzhao@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Itaru Kitayama <itaru.kitayama@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Hubbard <jhubbard@...dia.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v6 4/9] mm: thp: Introduce anon_orders and
 anon_always_mask sysfs files

On Fri, 29 Sep 2023 12:44:15 +0100 Ryan Roberts <ryan.roberts@....com> wrote:

> In preparation for adding support for anonymous large folios that are
> smaller than the PMD-size, introduce 2 new sysfs files that will be used
> to control the new behaviours via the transparent_hugepage interface.
> For now, the kernel still only supports PMD-order anonymous THP, so when
> reading back anon_orders, it will reflect that. Therefore there are no
> behavioural changes intended here.

powerpc strikes again.  ARCH=powerpc allmodconfig:


In file included from ./include/linux/bits.h:6,
                 from ./include/linux/ratelimit_types.h:5,
                 from ./include/linux/printk.h:9,
                 from ./include/asm-generic/bug.h:22,
                 from ./arch/powerpc/include/asm/bug.h:116,
                 from ./include/linux/bug.h:5,
                 from ./include/linux/mmdebug.h:5,
                 from ./include/linux/mm.h:6,
                 from mm/huge_memory.c:8:
./include/vdso/bits.h:7:33: error: initializer element is not constant
    7 | #define BIT(nr)                 (UL(1) << (nr))
      |                                 ^
mm/huge_memory.c:77:47: note: in expansion of macro 'BIT'
   77 | unsigned int huge_anon_orders __read_mostly = BIT(PMD_ORDER);
      |                                               ^~~

We keep tripping over this.  I wish there was a way to fix it.



Style whine: an all-caps identifier is supposed to be a constant,
dammit.

	#define PTE_INDEX_SIZE  __pte_index_size

Nope.



I did this:

--- a/mm/huge_memory.c~mm-thp-introduce-anon_orders-and-anon_always_mask-sysfs-files-fix
+++ a/mm/huge_memory.c
@@ -74,7 +74,7 @@ static unsigned long deferred_split_scan
 static atomic_t huge_zero_refcount;
 struct page *huge_zero_page __read_mostly;
 unsigned long huge_zero_pfn __read_mostly = ~0UL;
-unsigned int huge_anon_orders __read_mostly = BIT(PMD_ORDER);
+unsigned int huge_anon_orders __read_mostly;
 static unsigned int huge_anon_always_mask __read_mostly;
 
 /**
@@ -528,6 +528,9 @@ static int __init hugepage_init_sysfs(st
 {
 	int err;
 
+	/* powerpc's PMD_ORDER isn't a compile-time constant */
+	huge_anon_orders = BIT(PMD_ORDER);
+
 	*hugepage_kobj = kobject_create_and_add("transparent_hugepage", mm_kobj);
 	if (unlikely(!*hugepage_kobj)) {
 		pr_err("failed to create transparent hugepage kobject\n");
_


I assume this is set up early enough.

I don't know why powerpc's PTE_INDEX_SIZE is variable.  Hopefully it
has been set up by this time and it won't get altered.