linux-kernel - [PATCH 1/7] vfs: dcache: move hashlen_hash() from callers into d

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Message-ID: <20240610204821.230388-2-torvalds@linux-foundation.org>
Date: Mon, 10 Jun 2024 13:48:15 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Peter Anvin <hpa@...or.com>,
	Ingo Molnar <mingo@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	linux-arm-kernel@...ts.infradead.org,
	linux-arch <linux-arch@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 1/7] vfs: dcache: move hashlen_hash() from callers into d_hash()

Both __d_lookup_rcu() and __d_lookup_rcu_op_compare() have the full
'name_hash' value of the qstr that they want to look up, and mask it off
to just the low 32-bit hash before calling down to d_hash().

Other callers just load the 32-bit hash and pass it as the argument.

If we move the masking into d_hash() itself, it simplifies the two
callers that currently do the masking, and is a no-op for the other
cases.  It doesn't actually change the generated code since the compiler
will inline d_hash() and see that the end result is the same.

[ Technically, since the parse tree changes, the code generation may not
  be 100% the same, and for me on x86-64, this does result in gcc
  switching the operands around for one 'cmpl' instruction. So not
  necessarily the exact same code generation, but equivalent ]

However, this does encapsulate the 'd_hash()' operation more, and makes
the shift operation in particular be a "shift 32 bits right, return full
word".  Which matches the instruction semantics on both x86-64 and arm64
better, since a 32-bit shift will clear the upper bits.

That makes the next step of introducing a "shift by runtime constant"
more obvious and generates the shift with no extraneous type masking.

Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
---
 fs/dcache.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 407095188f83..8b4382f5c99d 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -100,9 +100,9 @@ static unsigned int d_hash_shift __ro_after_init;

 static struct hlist_bl_head *dentry_hashtable __ro_after_init;

-static inline struct hlist_bl_head *d_hash(unsigned int hash)
+static inline struct hlist_bl_head *d_hash(unsigned long hashlen)
 {
-	return dentry_hashtable + (hash >> d_hash_shift);
+	return dentry_hashtable + ((u32)hashlen >> d_hash_shift);
 }

 #define IN_LOOKUP_SHIFT 10
@@ -2104,7 +2104,7 @@ static noinline struct dentry *__d_lookup_rcu_op_compare(
 	unsigned *seqp)
 {
 	u64 hashlen = name->hash_len;
-	struct hlist_bl_head *b = d_hash(hashlen_hash(hashlen));
+	struct hlist_bl_head *b = d_hash(hashlen);
 	struct hlist_bl_node *node;
 	struct dentry *dentry;

@@ -2171,7 +2171,7 @@ struct dentry *__d_lookup_rcu(const struct dentry *parent,
 {
 	u64 hashlen = name->hash_len;
 	const unsigned char *str = name->name;
-	struct hlist_bl_head *b = d_hash(hashlen_hash(hashlen));
+	struct hlist_bl_head *b = d_hash(hashlen);
 	struct hlist_bl_node *node;
 	struct dentry *dentry;

-- 
2.45.1.209.gc6f12300df