Re: [PATCH RFC 09/13] mm/powerpc: Redefine pXd_huge() with pXd_leaf()

On Wed, Mar 06, 2024 at 11:56:56PM +1100, Michael Ellerman wrote:

peterx@redhat.com writes:

quoted

From: Peter Xu <peterx@redhat.com>

PowerPC book3s 4K mostly has the same definition on both, except pXd_huge()
constantly returns 0 for hash MMUs.  AFAICT that is fine to be removed,
because pXd_huge() reflects a hugetlb entry, while it's own hugetlb pgtable
lookup function (__find_linux_pte() shared by all powerpc code) already use
pXd_leaf() irrelevant of the MMU type.  It means pXd_leaf() should work all
fine with hash MMU pgtables or something could already went wrong.

Yes I think that's correct.

4K Hash MMU doesn't support any hugepage size at PMD or PUD level (the
geometry is wrong), so pmd/pud_huge() were written with that in mind,
ie. they are hard coded to return false.

But it should be OK to use pmd/pud_leaf(), they will actually look for
_PAGE_PTE, but it should never be set for 4K Hash.

See eg. arch/powerpc/include/asm/book3s/64/hash-4k.h:

static inline pmd_t hash__pmd_mkhuge(pmd_t pmd)
{
	BUG();
	return pmd;
}

Good to get confirmation on this, thanks, Michael.  These explanations also
look better than what I wrote, I'll amend the commit message.

Thanks for the help, I'll fix that.  I'm wondering when syzbot will start
to feed my series into the testers; I do still rely on those feedbacks on
compilation issues with such treewide changes, but so far I didn't yet
receive any reports.

I've also attached the new patch directly here in case of any further
comment.

Thanks,

==========8<===========
From 9e75aef2141170f241577e7786aaa4bbbfd93360 Mon Sep 17 00:00:00 2001
From: Peter Xu <peterx@redhat.com>
Date: Wed, 6 Mar 2024 14:49:48 +0800
Subject: [PATCH] mm/powerpc: Redefine pXd_huge() with pXd_leaf()

PowerPC book3s 4K mostly has the same definition on both, except pXd_huge()
constantly returns 0 for hash MMUs.  As Michael Ellerman pointed out [1],
it is safe to check _PAGE_PTE on hash MMUs, as the bit will never be set so
it will keep returning false.

As a reference, __p[mu]d_mkhuge() will trigger a BUG_ON trying to create
such huge mappings for 4K hash MMUs.  Meanwhile, the major powerpc hugetlb
pgtable walker __find_linux_pte(), already used pXd_leaf() to check hugetlb
mappings.

The goal should be that we will have one API pXd_leaf() to detect all kinds
of huge mappings.  AFAICT we need to use the pXd_leaf() impl (rather than
pXd_huge() ones) to make sure ie. THPs on hash MMU will also return true.

This helps to simplify a follow up patch to drop pXd_huge() treewide.

NOTE: *_leaf() definition need to be moved before the inclusion of
asm/book3s/64/pgtable-4k.h, which defines pXd_huge() with it.

[1] https://lore.kernel.org/r/87v85zo6w7.fsf@mail.lhotse (local)

Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Christophe Leroy <redacted>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>
Cc: "Naveen N. Rao" <redacted>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 .../include/asm/book3s/64/pgtable-4k.h        | 14 ++--------
 arch/powerpc/include/asm/book3s/64/pgtable.h  | 27 +++++++++----------
 2 files changed, 14 insertions(+), 27 deletions(-)

diff --git a/arch/powerpc/include/asm/book3s/64/pgtable-4k.h b/arch/powerpc/include/asm/book3s/64/pgtable-4k.h
index 48f21820afe2..92545981bb49 100644
--- a/arch/powerpc/include/asm/book3s/64/pgtable-4k.h
+++ b/arch/powerpc/include/asm/book3s/64/pgtable-4k.h

@@ -8,22 +8,12 @@
 #ifdef CONFIG_HUGETLB_PAGE
 static inline int pmd_huge(pmd_t pmd)
 {
-	/*
-	 * leaf pte for huge page
-	 */
-	if (radix_enabled())
-		return !!(pmd_raw(pmd) & cpu_to_be64(_PAGE_PTE));
-	return 0;
+	return pmd_leaf(pmd);
 }
 
 static inline int pud_huge(pud_t pud)
 {
-	/*
-	 * leaf pte for huge page
-	 */
-	if (radix_enabled())
-		return !!(pud_raw(pud) & cpu_to_be64(_PAGE_PTE));
-	return 0;
+	return pud_leaf(pud);
 }
 
 /*

diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
index df66dce8306f..fd7180fded75 100644
--- a/arch/powerpc/include/asm/book3s/64/pgtable.h
+++ b/arch/powerpc/include/asm/book3s/64/pgtable.h

@@ -262,6 +262,18 @@ extern unsigned long __kernel_io_end;
 
 extern struct page *vmemmap;
 extern unsigned long pci_io_base;
+
+#define pmd_leaf pmd_leaf
+static inline bool pmd_leaf(pmd_t pmd)
+{
+	return !!(pmd_raw(pmd) & cpu_to_be64(_PAGE_PTE));
+}
+
+#define pud_leaf pud_leaf
+static inline bool pud_leaf(pud_t pud)
+{
+	return !!(pud_raw(pud) & cpu_to_be64(_PAGE_PTE));
+}
 #endif /* __ASSEMBLY__ */
 
 #include <asm/book3s/64/hash.h>

@@ -1436,20 +1448,5 @@ static inline bool is_pte_rw_upgrade(unsigned long old_val, unsigned long new_va
 	return false;
 }
 
-/*
- * Like pmd_huge(), but works regardless of config options
- */
-#define pmd_leaf pmd_leaf
-static inline bool pmd_leaf(pmd_t pmd)
-{
-	return !!(pmd_raw(pmd) & cpu_to_be64(_PAGE_PTE));
-}
-
-#define pud_leaf pud_leaf
-static inline bool pud_leaf(pud_t pud)
-{
-	return !!(pud_raw(pud) & cpu_to_be64(_PAGE_PTE));
-}
-
 #endif /* __ASSEMBLY__ */
 #endif /* _ASM_POWERPC_BOOK3S_64_PGTABLE_H_ */

-- 
2.44.0

-- 
Peter Xu