fuse: simplify logic in fuse_notify_store() and fuse_retrieve()

Simplify the folio parsing logic in fuse_notify_store() and
fuse_retrieve().

In particular, calculate the index by tracking pos, which allows us to
remove calculating nr_pages, and use "pos" in place of outarg's offset
field.

Suggested-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Author: joannekoong

fs/fuse/dev.c

@@ -1767,10 +1767,9 @@ static int fuse_notify_store(struct fuse_conn *fc, unsigned int size,
 	struct address_space *mapping;
 	u64 nodeid;
 	int err;
-	pgoff_t index;
-	unsigned int offset;
 	unsigned int num;
 	loff_t file_size;
+	loff_t pos;
 	loff_t end;
 
 	if (size < sizeof(outarg))
@@ -1787,7 +1786,8 @@ static int fuse_notify_store(struct fuse_conn *fc, unsigned int size,
 		return -EINVAL;
 
 	nodeid = outarg.nodeid;
-	num = min(outarg.size, MAX_LFS_FILESIZE - outarg.offset);
+	pos = outarg.offset;
+	num = min(outarg.size, MAX_LFS_FILESIZE - pos);
 
 	down_read(&fc->killsb);
 
@@ -1797,10 +1797,8 @@ static int fuse_notify_store(struct fuse_conn *fc, unsigned int size,
 		goto out_up_killsb;
 
 	mapping = inode->i_mapping;
-	index = outarg.offset >> PAGE_SHIFT;
-	offset = outarg.offset & ~PAGE_MASK;
 	file_size = i_size_read(inode);
-	end = outarg.offset + num;
+	end = pos + num;
 	if (end > file_size) {
 		file_size = end;
 		fuse_write_update_attr(inode, file_size, num);
@@ -1810,19 +1808,18 @@ static int fuse_notify_store(struct fuse_conn *fc, unsigned int size,
 		struct folio *folio;
 		unsigned int folio_offset;
 		unsigned int nr_bytes;
-		unsigned int nr_pages;
+		pgoff_t index = pos >> PAGE_SHIFT;
 
 		folio = filemap_grab_folio(mapping, index);
 		err = PTR_ERR(folio);
 		if (IS_ERR(folio))
 			goto out_iput;
 
-		folio_offset = ((index - folio->index) << PAGE_SHIFT) + offset;
+		folio_offset = offset_in_folio(folio, pos);
 		nr_bytes = min(num, folio_size(folio) - folio_offset);
-		nr_pages = (offset + nr_bytes + PAGE_SIZE - 1) >> PAGE_SHIFT;
 
 		err = fuse_copy_folio(cs, &folio, folio_offset, nr_bytes, 0);
-		if (!folio_test_uptodate(folio) && !err && offset == 0 &&
+		if (!folio_test_uptodate(folio) && !err && folio_offset == 0 &&
 		    (nr_bytes == folio_size(folio) || file_size == end)) {
 			folio_zero_segment(folio, nr_bytes, folio_size(folio));
 			folio_mark_uptodate(folio);
@@ -1833,9 +1830,8 @@ static int fuse_notify_store(struct fuse_conn *fc, unsigned int size,
 		if (err)
 			goto out_iput;
 
+		pos += nr_bytes;
 		num -= nr_bytes;
-		offset = 0;
-		index += nr_pages;
 	}
 
 	err = 0;
@@ -1867,7 +1863,6 @@ static int fuse_retrieve(struct fuse_mount *fm, struct inode *inode,
 {
 	int err;
 	struct address_space *mapping = inode->i_mapping;
-	pgoff_t index;
 	loff_t file_size;
 	unsigned int num;
 	unsigned int offset;
@@ -1878,15 +1873,16 @@ static int fuse_retrieve(struct fuse_mount *fm, struct inode *inode,
 	size_t args_size = sizeof(*ra);
 	struct fuse_args_pages *ap;
 	struct fuse_args *args;
+	loff_t pos = outarg->offset;
 
-	offset = outarg->offset & ~PAGE_MASK;
+	offset = offset_in_page(pos);
 	file_size = i_size_read(inode);
 
 	num = min(outarg->size, fc->max_write);
-	if (outarg->offset > file_size)
+	if (pos > file_size)
 		num = 0;
-	else if (num > file_size - outarg->offset)
-		num = file_size - outarg->offset;
+	else if (num > file_size - pos)
+		num = file_size - pos;
 
 	num_pages = (num + offset + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	num_pages = min(num_pages, fc->max_pages);
@@ -1909,31 +1905,27 @@ static int fuse_retrieve(struct fuse_mount *fm, struct inode *inode,
 	args->in_pages = true;
 	args->end = fuse_retrieve_end;
 
-	index = outarg->offset >> PAGE_SHIFT;
-
 	while (num && ap->num_folios < num_pages) {
 		struct folio *folio;
 		unsigned int folio_offset;
 		unsigned int nr_bytes;
-		unsigned int nr_pages;
+		pgoff_t index = pos >> PAGE_SHIFT;
 
 		folio = filemap_get_folio(mapping, index);
 		if (IS_ERR(folio))
 			break;
 
-		folio_offset = ((index - folio->index) << PAGE_SHIFT) + offset;
+		folio_offset = offset_in_folio(folio, pos);
 		nr_bytes = min(folio_size(folio) - folio_offset, num);
-		nr_pages = (offset + nr_bytes + PAGE_SIZE - 1) >> PAGE_SHIFT;
 
 		ap->folios[ap->num_folios] = folio;
 		ap->descs[ap->num_folios].offset = folio_offset;
 		ap->descs[ap->num_folios].length = nr_bytes;
 		ap->num_folios++;
 
-		offset = 0;
+		pos += nr_bytes;
 		num -= nr_bytes;
 		total_len += nr_bytes;
-		index += nr_pages;
 	}
 	ra->inarg.offset = outarg->offset;
 	ra->inarg.size = total_len;