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Date: Fri, 22 Jul 2022 16:19:32 +0300
From: Nikolay Borisov <nborisov@...e.com>
To: viro@...iv.linux.org.uk
Cc: linux-kernel@...r.kernel.org, agruenba@...hat.com,
Nikolay Borisov <nborisov@...e.com>
Subject: [PATCH] iov_iter: Microoptimize fault_in_iov_iter_(writable|readable)
Inverting the condition inside the 2 functions results in reduced nesting
as well as saving 20 bytes of generated codeu as well as eliminating a mandatory
branch when a proper userspace iov iter is passed:
add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-20 (-20)
Function old new delta
fault_in_iov_iter_writeable 131 121 -10
fault_in_iov_iter_readable 131 121 -10
That's because in the original version the compiler duplicates the
function epilogue right after the initial branch, resulting in the
following code:
0x000000000000081c <+12>: je 0x828 <fault_in_iov_iter_readable+24>
0x000000000000081e <+14>: pop %rbx
0x000000000000081f <+15>: mov %r12,%rax
0x0000000000000822 <+18>: pop %rbp
0x0000000000000823 <+19>: pop %r12
0x0000000000000825 <+21>: pop %r13
0x0000000000000827 <+23>: retq
0x0000000000000828 <+24>: cmp %rsi,0x10(%rdi) ;loop begins
There is always a branch penalty before actual executioni begins,
whilst with this patch the generated code is:
0x00000000000004ac <+12>: jne 0x507 <fault_in_iov_iter_readable+103>
0x00000000000004ae <+14>: cmp %rsi,0x10(%rdi)
The branch is taken only when the passed in iter is not an iovec
Signed-off-by: Nikolay Borisov <nborisov@...e.com>
---
lib/iov_iter.c | 76 +++++++++++++++++++++++++-------------------------
1 file changed, 38 insertions(+), 38 deletions(-)
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 0b64695ab632..897148d9cd12 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -443,26 +443,26 @@ static size_t copy_page_to_iter_pipe(struct page *page, size_t offset, size_t by
*/
size_t fault_in_iov_iter_readable(const struct iov_iter *i, size_t size)
{
- if (iter_is_iovec(i)) {
- size_t count = min(size, iov_iter_count(i));
- const struct iovec *p;
- size_t skip;
-
- size -= count;
- for (p = i->iov, skip = i->iov_offset; count; p++, skip = 0) {
- size_t len = min(count, p->iov_len - skip);
- size_t ret;
-
- if (unlikely(!len))
- continue;
- ret = fault_in_readable(p->iov_base + skip, len);
- count -= len - ret;
- if (ret)
- break;
- }
- return count + size;
+ size_t count = min(size, iov_iter_count(i));
+ const struct iovec *p;
+ size_t skip;
+
+ if (!iter_is_iovec(i))
+ return 0;
+
+ size -= count;
+ for (p = i->iov, skip = i->iov_offset; count; p++, skip = 0) {
+ size_t len = min(count, p->iov_len - skip);
+ size_t ret;
+
+ if (unlikely(!len))
+ continue;
+ ret = fault_in_readable(p->iov_base + skip, len);
+ count -= len - ret; // reduce count by the number faulted in
+ if (ret)
+ break;
}
- return 0;
+ return count + size;
}
EXPORT_SYMBOL(fault_in_iov_iter_readable);
@@ -482,26 +482,26 @@ EXPORT_SYMBOL(fault_in_iov_iter_readable);
*/
size_t fault_in_iov_iter_writeable(const struct iov_iter *i, size_t size)
{
- if (iter_is_iovec(i)) {
- size_t count = min(size, iov_iter_count(i));
- const struct iovec *p;
- size_t skip;
-
- size -= count;
- for (p = i->iov, skip = i->iov_offset; count; p++, skip = 0) {
- size_t len = min(count, p->iov_len - skip);
- size_t ret;
-
- if (unlikely(!len))
- continue;
- ret = fault_in_safe_writeable(p->iov_base + skip, len);
- count -= len - ret;
- if (ret)
- break;
- }
- return count + size;
+ size_t count = min(size, iov_iter_count(i));
+ const struct iovec *p;
+ size_t skip;
+
+ if (!iter_is_iovec(i))
+ return 0;
+
+ size -= count;
+ for (p = i->iov, skip = i->iov_offset; count; p++, skip = 0) {
+ size_t len = min(count, p->iov_len - skip);
+ size_t ret;
+
+ if (unlikely(!len))
+ continue;
+ ret = fault_in_safe_writeable(p->iov_base + skip, len);
+ count -= len - ret;
+ if (ret)
+ break;
}
- return 0;
+ return count + size;
}
EXPORT_SYMBOL(fault_in_iov_iter_writeable);
--
2.25.1
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