linux-kernel - [PATCH] iov_iter: streamline iovec/bvec alignment iteration

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Date: Tue, 23 Jan 2024 12:49:18 -0700
From: Jens Axboe <axboe@...nel.dk>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>
Subject: [PATCH] iov_iter: streamline iovec/bvec alignment iteration

Rewrite iov_iter_aligned_iovec() and iov_iter_aligned_bvec() to be both
easier to read, and also significantly more compact in terms of
generated code. This saves 178 bytes of text on x86-64 for me (with
clang-18) and 136 bytes on arm64 (with gcc-13).

In profiles, also saves a bit of time as well for the same workload:

     0.81%     -0.18%  [kernel.vmlinux]  [k] iov_iter_aligned_bvec
     0.48%     -0.09%  [kernel.vmlinux]  [k] iov_iter_is_aligned

which is a nice side benefit as well.

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index e0aa6b440ca5..2fcc47d822e3 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -714,12 +714,11 @@ EXPORT_SYMBOL(iov_iter_discard);
 static bool iov_iter_aligned_iovec(const struct iov_iter *i, unsigned addr_mask,
 				   unsigned len_mask)
 {
+	const struct iovec *iov = iter_iov(i);
 	size_t size = i->count;
 	size_t skip = i->iov_offset;
-	unsigned k;
 
-	for (k = 0; k < i->nr_segs; k++, skip = 0) {
-		const struct iovec *iov = iter_iov(i) + k;
+	do {
 		size_t len = iov->iov_len - skip;
 
 		if (len > size)
@@ -729,34 +728,36 @@ static bool iov_iter_aligned_iovec(const struct iov_iter *i, unsigned addr_mask,
 		if ((unsigned long)(iov->iov_base + skip) & addr_mask)
 			return false;
 
+		iov++;
 		size -= len;
-		if (!size)
-			break;
-	}
+		skip = 0;
+	} while (size);
+
 	return true;
 }
 
 static bool iov_iter_aligned_bvec(const struct iov_iter *i, unsigned addr_mask,
 				  unsigned len_mask)
 {
-	size_t size = i->count;
+	const struct bio_vec *bvec = i->bvec;
 	unsigned skip = i->iov_offset;
-	unsigned k;
+	size_t size = i->count;
 
-	for (k = 0; k < i->nr_segs; k++, skip = 0) {
-		size_t len = i->bvec[k].bv_len - skip;
+	do {
+		size_t len = bvec->bv_len;
 
 		if (len > size)
 			len = size;
 		if (len & len_mask)
 			return false;
-		if ((unsigned long)(i->bvec[k].bv_offset + skip) & addr_mask)
+		if ((unsigned long)(bvec->bv_offset + skip) & addr_mask)
 			return false;
 
+		bvec++;
 		size -= len;
-		if (!size)
-			break;
-	}
+		skip = 0;
+	} while (size);
+
 	return true;
 }
 
-- 
Jens Axboe