linux-kernel - [POC 12/12] fs/dcache.c: use rai_bucket

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Date:   Thu, 18 Oct 2018 00:33:32 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: [POC 12/12] fs/dcache.c: use rai_bucket_shift for dentry hashtable

Before this, the disassembly of __d_lookup_rcu begins

   0x2d10 <__d_lookup_rcu>:     push   %r15
   0x2d12 <__d_lookup_rcu+2>:   push   %r14
   0x2d14 <__d_lookup_rcu+4>:   push   %r13
   0x2d16 <__d_lookup_rcu+6>:   push   %r12
   0x2d18 <__d_lookup_rcu+8>:   push   %rbp
   0x2d19 <__d_lookup_rcu+9>:   push   %rbx
   0x2d1a <__d_lookup_rcu+10>:  sub    $0x18,%rsp
   0x2d1e <__d_lookup_rcu+14>:  mov    (%rsi),%r12
   0x2d21 <__d_lookup_rcu+17>:  mov    0x0(%rip),%ecx        # 0x2d27 <__d_lookup_rcu+23>
   0x2d27 <__d_lookup_rcu+23>:  mov    0x8(%rsi),%r13
   0x2d2b <__d_lookup_rcu+27>:  mov    %r12d,%eax
   0x2d2e <__d_lookup_rcu+30>:  shr    %cl,%eax
   0x2d30 <__d_lookup_rcu+32>:  mov    0x0(%rip),%rcx        # 0x2d37 <__d_lookup_rcu+39>
   0x2d37 <__d_lookup_rcu+39>:  lea    (%rcx,%rax,8),%rax
   0x2d3b <__d_lookup_rcu+43>:  mov    (%rax),%rbx
   0x2d3e <__d_lookup_rcu+46>:  and    $0xfffffffffffffffe,%rbx
   0x2d42 <__d_lookup_rcu+50>:  je     0x2df3 <__d_lookup_rcu+227>
   0x2d48 <__d_lookup_rcu+56>:  mov    %r12,%rax
   0x2d4b <__d_lookup_rcu+59>:  mov    %r12d,%r10d
   0x2d4e <__d_lookup_rcu+62>:  mov    %rdx,%r15

After this, and after the patching of the run-time values of
dentry_hashtable and d_hash_shift, gdb on /proc/kcore says that we now
have

   0xffffffff8902e8d0:  push   %r15
   0xffffffff8902e8d2:  push   %r14
   0xffffffff8902e8d4:  push   %r13
   0xffffffff8902e8d6:  push   %r12
   0xffffffff8902e8d8:  push   %rbp
   0xffffffff8902e8d9:  push   %rbx
   0xffffffff8902e8da:  sub    $0x18,%rsp
   0xffffffff8902e8de:  mov    (%rsi),%r12
   0xffffffff8902e8e1:  mov    0x8(%rsi),%r13
   0xffffffff8902e8e5:  mov    %r12d,%eax
   0xffffffff8902e8e8:  movabs $0xffff959b80007000,%rcx
   0xffffffff8902e8f2:  shr    $0x12,%eax
   0xffffffff8902e8f5:  lea    (%rcx,%rax,8),%rcx
   0xffffffff8902e8f9:  mov    (%rcx),%rbx
   0xffffffff8902e8fc:  and    $0xfffffffffffffffe,%rbx
   0xffffffff8902e900:  je     0xffffffff8902e9b1
   0xffffffff8902e906:  mov    %r12,%rax
   0xffffffff8902e909:  mov    %r12d,%r10d
   0xffffffff8902e90c:  mov    %rdx,%r15

The shr $0x12, %eax is consistent with

[    0.300676] Dentry cache hash table entries: 16384 (order: 5, 131072 bytes)

and nothing seems to explode.

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 fs/dcache.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 1d54dfb38c9d..226298c3a599 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -100,7 +100,7 @@ static struct hlist_bl_head *dentry_hashtable __read_mostly;
 
 static inline struct hlist_bl_head *d_hash(unsigned int hash)
 {
-	return dentry_hashtable + (hash >> d_hash_shift);
+	return rai_bucket_shift(dentry_hashtable, d_hash_shift, hash);
 }
 
 #define IN_LOOKUP_SHIFT 10
-- 
2.19.1.6.gbde171bbf5