[PATCH 4.14 089/189] : fix end_name_hash() for 64bit long

Greg Kroah-Hartman
2018-06-18T08:33:21Z

From: Amir Goldstein <amir73il@gmail.com>

[ Upstream commit 19b9ad67310ed2f685062a00aec602bec33835f0 ]

The comment claims that this helper will try not to loose bits, but for
64bit long it looses the high bits before hashing 64bit long into 32bit
int. Use the helper hash_long() to do the right thing for 64bit long.
For 32bit long, there is no change.

All the callers of end_name_hash() either assign the result to
qstr->hash, which is u32 or return the result as an int value (e.g.
full_name_hash()). Change the helper return type to int to conform to
its users.

[ It took me a while to apply this, because my initial reaction to it
was - incorrectly - that it could make for slower code.

After having looked more at it, I take back all my complaints about
the patch, Amir was right and I was mis-reading things or just being
stupid.

I also don't worry too much about the possible performance impact of
this on 64-bit, since most architectures that actually care about
performance end up not using this very much (the dcache code is the
most performance-critical, but the word-at-a-time case uses its own
hashing anyway).

So this ends up being mostly used for filesystems that do their own
degraded hashing (usually because they want a case-insensitive
comparison function).

A _tiny_ worry remains, in that not everybody uses DCACHE_WORD_ACCESS,
and then this potentially makes things more expensive on 64-bit
architectures with slow or lacking multipliers even for the normal
case.

That said, realistically the only such architecture I can think of is
PA-RISC. Nobody really cares about performance on that, it's more of a
"look ma, I've got warts^W an odd machine" platform.

So the patch is fine, and all my initial worries were just misplaced
from not looking at this properly. - Linus ]

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/linux/stringhash.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/include/linux/stringhash.h
+++ b/include/linux/stringhash.h
@@ -50,9 +50,9 @@ partial_name_hash(unsigned long c, unsig
* losing bits). This also has the property (wanted by the dcache)
* that the msbits make a good hash table index.
*/
-static inline unsigned long end_name_hash(unsigned long hash)
+static inline unsigned int end_name_hash(unsigned long hash)
{
- return __hash_32((unsigned int)hash);
+ return hash_long(hash, 32);
}

/*