Re: [RFC] what to do with IOCB_DSYNC?

[Jens Axboe <axboe@kernel.dk>]

On 5/22/22 7:22 PM, Jens Axboe wrote:
> On 5/22/22 6:42 PM, Al Viro wrote:
>> On Sun, May 22, 2022 at 02:03:35PM -0600, Jens Axboe wrote:
>>
>>> Right, I'm saying it's not _immediately_ clear which cases are what when
>>> reading the code.
>>>
>>>> up a while ago.  And no, turning that into indirect calls ended up with
>>>> arseloads of overhead, more's the pity...
>>>
>>> It's a shame, since indirect calls make for nicer code, but it's always
>>> been slower and these days even more so.
>>>
>>>> Anyway, at the moment I have something that builds; hadn't tried to
>>>> boot it yet.
>>>
>>> Nice!
>>
>> Boots and survives LTP and xfstests...  Current variant is in
>> vfs.git#work.iov_iter (head should be at 27fa77a9829c).  I have *not*
>> looked into the code generation in primitives; the likely/unlikely on
>> those cascades of ifs need rethinking.
> 
> I noticed too. Haven't fiddled much in iov_iter.c, but for uio.h I had
> the below. iov_iter.c is a worse "offender" though, with 53 unlikely and
> 22 likely annotations...

Here it is...


diff --git a/include/linux/uio.h b/include/linux/uio.h
index 6570b688ed39..52baa3c89505 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -163,19 +163,17 @@ static inline size_t copy_folio_to_iter(struct folio *folio, size_t offset,
 static __always_inline __must_check
 size_t copy_to_iter(const void *addr, size_t bytes, struct iov_iter *i)
 {
-	if (unlikely(!check_copy_size(addr, bytes, true)))
-		return 0;
-	else
+	if (check_copy_size(addr, bytes, true))
 		return _copy_to_iter(addr, bytes, i);
+	return 0;
 }
 
 static __always_inline __must_check
 size_t copy_from_iter(void *addr, size_t bytes, struct iov_iter *i)
 {
-	if (unlikely(!check_copy_size(addr, bytes, false)))
-		return 0;
-	else
+	if (check_copy_size(addr, bytes, false))
 		return _copy_from_iter(addr, bytes, i);
+	return 0;
 }
 
 static __always_inline __must_check
@@ -191,10 +189,9 @@ bool copy_from_iter_full(void *addr, size_t bytes, struct iov_iter *i)
 static __always_inline __must_check
 size_t copy_from_iter_nocache(void *addr, size_t bytes, struct iov_iter *i)
 {
-	if (unlikely(!check_copy_size(addr, bytes, false)))
-		return 0;
-	else
+	if (check_copy_size(addr, bytes, false))
 		return _copy_from_iter_nocache(addr, bytes, i);
+	return 0;
 }
 
 static __always_inline __must_check

-- 
Jens Axboe