FAILED: patch "[PATCH] teach copy_page_to_iter() to handle compound pages" failed to apply to 5.12-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.12-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 08aa64796016cb47b2ef3d0924653b4d944b0d65 Mon Sep 17 00:00:00 2001
From: Al Viro <viro@zeniv.linux.org.uk>
Date: Thu, 29 Apr 2021 20:42:25 -0400
Subject: [PATCH] teach copy_page_to_iter() to handle compound pages

	In situation when copy_page_to_iter() got a compound page the current
code would only work on systems with no CONFIG_HIGHMEM.  It *is* the majority
of real-world setups, or we would've drown in bug reports by now.  Still needs
fixing.

	Current variant works for solitary page; rename that to
__copy_page_to_iter() and turn the handling of compound pages into a loop over
subpages.

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 8f5ce5b1ff91..12fb04b23143 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -957,11 +957,9 @@ static inline bool page_copy_sane(struct page *page, size_t offset, size_t n)
 	return false;
 }
 
-size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
+static size_t __copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
 			 struct iov_iter *i)
 {
-	if (unlikely(!page_copy_sane(page, offset, bytes)))
-		return 0;
 	if (i->type & (ITER_BVEC | ITER_KVEC | ITER_XARRAY)) {
 		void *kaddr = kmap_atomic(page);
 		size_t wanted = copy_to_iter(kaddr + offset, bytes, i);
@@ -974,6 +972,30 @@ size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
 	else
 		return copy_page_to_iter_pipe(page, offset, bytes, i);
 }
+
+size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
+			 struct iov_iter *i)
+{
+	size_t res = 0;
+	if (unlikely(!page_copy_sane(page, offset, bytes)))
+		return 0;
+	page += offset / PAGE_SIZE; // first subpage
+	offset %= PAGE_SIZE;
+	while (1) {
+		size_t n = __copy_page_to_iter(page, offset,
+				min(bytes, (size_t)PAGE_SIZE - offset), i);
+		res += n;
+		bytes -= n;
+		if (!bytes || !n)
+			break;
+		offset += n;
+		if (offset == PAGE_SIZE) {
+			page++;
+			offset = 0;
+		}
+	}
+	return res;
+}
 EXPORT_SYMBOL(copy_page_to_iter);
 
 size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,