Re: [PATCH] iov_iter: streamline iovec/bvec alignment iteration

[Jens Axboe <axboe@kernel.dk>]

On 1/24/24 5:07 AM, David Laight wrote:
> From: Jens Axboe
>> Sent: 23 January 2024 19:49
>>
>> 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).
>>
> ...
>> 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);
> ...
>> -	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;
> 
> Is i->nr_segs allowed to be zero?

Not if size is not zero.

> The old code (seemed to) check for zero.
> The new version will go horribly wrong.

In other spots we do check both segments and size, but not that
nr_segments is non-zero to begin with. So I think this revised version
may be better. Saves 176 bytes of text on arm64 for me, and 126 bytes on
x86-64. I'll send out a v3.


diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index e0aa6b440ca5..5e34639c5d1e 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -714,12 +714,12 @@ 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;
+	unsigned long nr_segs = i->nr_segs;
 
-	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 +729,37 @@ 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 && --nr_segs);
+
 	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;
+	unsigned long nr_segs = i->nr_segs;
 
-	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 && --nr_segs);
+
 	return true;
 }
 
@@ -800,13 +803,13 @@ EXPORT_SYMBOL_GPL(iov_iter_is_aligned);
 
 static unsigned long iov_iter_alignment_iovec(const struct iov_iter *i)
 {
+	const struct iovec *iov = iter_iov(i);
 	unsigned long res = 0;
 	size_t size = i->count;
 	size_t skip = i->iov_offset;
-	unsigned k;
+	unsigned long nr_segs = i->nr_segs;
 
-	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) {
 			res |= (unsigned long)iov->iov_base + skip;
@@ -814,30 +817,33 @@ static unsigned long iov_iter_alignment_iovec(const struct iov_iter *i)
 				len = size;
 			res |= len;
 			size -= len;
-			if (!size)
-				break;
 		}
-	}
+		iov++;
+		skip = 0;
+	} while (size && --nr_segs);
+
 	return res;
 }
 
 static unsigned long iov_iter_alignment_bvec(const struct iov_iter *i)
 {
+	const struct bio_vec *bvec = i->bvec;
 	unsigned res = 0;
 	size_t size = i->count;
 	unsigned skip = i->iov_offset;
-	unsigned k;
+	unsigned long nr_segs = i->nr_segs;
 
-	for (k = 0; k < i->nr_segs; k++, skip = 0) {
-		size_t len = i->bvec[k].bv_len - skip;
-		res |= (unsigned long)i->bvec[k].bv_offset + skip;
+	do {
+		size_t len = bvec->bv_len - skip;
+		res |= (unsigned long)bvec->bv_offset + skip;
 		if (len > size)
 			len = size;
 		res |= len;
+		bvec++;
 		size -= len;
-		if (!size)
-			break;
-	}
+		skip = 0;
+	} while (size && --nr_segs);
+
 	return res;
 }
 

-- 
Jens Axboe