Re: [PATCH v6 4/9] mm: thp: Introduce anon_orders and anon_always_mask sysfs files
From: Michael Ellerman <mpe@ellerman.id.au>
Date: 2023-10-07 23:37:34
Also in:
linux-arm-kernel, linux-mm, lkml
Andrew Morton [off-list ref] writes:
On Fri, 29 Sep 2023 12:44:15 +0100 Ryan Roberts [off-list ref] wrote:quoted
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.
I can't think of any solution, other than ripping the code out. To catch it earlier we'd need a generic compile-time test that all values derived from the page table geometry are only used in places that don't require a constant. I can't think of a way to write a test for that. Or submitters could compile-test for powerpc - one can dream :D
Style whine: an all-caps identifier is supposed to be a constant, dammit. #define PTE_INDEX_SIZE __pte_index_size Nope.
I agree it's ugly. It was done that way because PTE_INDEX_SIZE used to be constant, and still is for 32-bit PPC and 64-bit Book3E PPC. We could rename PTE_INDEX_SIZE itself, but we'd still have eg. PTE_TABLE_SIZE which is used in generic code, and which would be sometimes constant and sometimes not for different powerpc subarches.
quoted hunk ↗ jump to hunk
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.
Yes it should be.
I don't know why powerpc's PTE_INDEX_SIZE is variable.
To allow a single vmlinux to boot using either the Hashed Page Table MMU, or Radix Tree MMU, which have different page table geometry. That's a pretty crucial feature for distros, so that they can build a single kernel to boot on Power8/9/10. cheers