Re: Optimizing small reads

[Linus Torvalds <torvalds@linux-foundation.org>]

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On Tue, 7 Oct 2025 at 15:35, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> But I think I'll try to boot it next. Wish me luck.

Bah. It boots - after you fix the stupid double increment of 'already_copied'.

I didn't remove the update inside the loop when I made it update it
after the loop.

So here's the slightly fixed patch that actually does boot - and that
I'm running right now. But I wouldn't call it exactly "tested".

Caveat patchor.

          Linus

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 mm/filemap.c | 33 +++++++++++++++++++++++++--------
 1 file changed, 25 insertions(+), 8 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 60a7b9275741..ba11f018ca6b 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2792,20 +2792,37 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
 	 * any compiler initialization would be pointless since this
 	 * can fill it will garbage.
 	 */
-	if (iov_iter_count(iter) <= sizeof(area)) {
+	if (iov_iter_count(iter) <= PAGE_SIZE) {
 		size_t count = iov_iter_count(iter);
+		size_t fast_read = 0;
 
 		/* Let's see if we can just do the read under RCU */
 		rcu_read_lock();
-		count = filemap_fast_read(mapping, iocb->ki_pos, area.buffer, count);
+		pagefault_disable();
+		do {
+			size_t copied = min(count, sizeof(area));
+
+			copied = filemap_fast_read(mapping, iocb->ki_pos, area.buffer, copied);
+			if (!copied)
+				break;
+			copied = copy_to_iter(area.buffer, copied, iter);
+			if (!copied)
+				break;
+			fast_read += copied;
+			iocb->ki_pos += copied;
+			count -= copied;
+		} while (count);
+		pagefault_enable();
 		rcu_read_unlock();
-		if (count) {
-			size_t copied = copy_to_iter(area.buffer, count, iter);
-			if (unlikely(!copied))
-				return already_read ? already_read : -EFAULT;
-			ra->prev_pos = iocb->ki_pos += copied;
+
+		if (fast_read) {
+			ra->prev_pos += fast_read;
+			already_read += fast_read;
 			file_accessed(filp);
-			return copied + already_read;
+
+			/* All done? */
+			if (!count)
+				return already_read;
 		}
 	}