iomap: stash iomap read ctx in the private field of iomap_iter

It's useful to get filesystem-specific information using the
existing private field in the @iomap_iter passed to iomap_{begin,end}
for advanced usage for iomap buffered reads, which is much like the
current iomap DIO.

For example, EROFS needs it to:

 - implement an efficient page cache sharing feature, since iomap
   needs to apply to anon inode page cache but we'd like to get the
   backing inode/fs instead, so filesystem-specific private data is
   needed to keep such information;

 - pass in both struct page * and void * for inline data to avoid
   kmap_to_page() usage (which is bogus).

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Link: https://patch.msgid.link/20260109102856.598531-2-lihongbo22@huawei.com
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Hongbo Li

fs/fuse/file.c

@@ -979,7 +979,7 @@ static int fuse_read_folio(struct file *file, struct folio *folio)
 		return -EIO;
 	}
 
-	iomap_read_folio(&fuse_iomap_ops, &ctx);
+	iomap_read_folio(&fuse_iomap_ops, &ctx, NULL);
 	fuse_invalidate_atime(inode);
 	return 0;
 }
@@ -1081,7 +1081,7 @@ static void fuse_readahead(struct readahead_control *rac)
 	if (fuse_is_bad(inode))
 		return;
 
-	iomap_readahead(&fuse_iomap_ops, &ctx);
+	iomap_readahead(&fuse_iomap_ops, &ctx, NULL);
 }
 
 static ssize_t fuse_cache_read_iter(struct kiocb *iocb, struct iov_iter *to)

fs/iomap/buffered-io.c

@@ -555,13 +555,14 @@ static int iomap_read_folio_iter(struct iomap_iter *iter,
 }
 
 void iomap_read_folio(const struct iomap_ops *ops,
-		struct iomap_read_folio_ctx *ctx)
+		struct iomap_read_folio_ctx *ctx, void *private)
 {
 	struct folio *folio = ctx->cur_folio;
 	struct iomap_iter iter = {
 		.inode		= folio->mapping->host,
 		.pos		= folio_pos(folio),
 		.len		= folio_size(folio),
+		.private	= private,
 	};
 	size_t bytes_submitted = 0;
 	int ret;
@@ -620,13 +621,14 @@ static int iomap_readahead_iter(struct iomap_iter *iter,
  * the filesystem to be reentered.
  */
 void iomap_readahead(const struct iomap_ops *ops,
-		struct iomap_read_folio_ctx *ctx)
+		struct iomap_read_folio_ctx *ctx, void *private)
 {
 	struct readahead_control *rac = ctx->rac;
 	struct iomap_iter iter = {
 		.inode	= rac->mapping->host,
 		.pos	= readahead_pos(rac),
 		.len	= readahead_length(rac),
+		.private = private,
 	};
 	size_t cur_bytes_submitted;
 

include/linux/iomap.h

@@ -341,9 +341,9 @@ ssize_t iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *from,
 		const struct iomap_ops *ops,
 		const struct iomap_write_ops *write_ops, void *private);
 void iomap_read_folio(const struct iomap_ops *ops,
-		struct iomap_read_folio_ctx *ctx);
+		struct iomap_read_folio_ctx *ctx, void *private);
 void iomap_readahead(const struct iomap_ops *ops,
-		struct iomap_read_folio_ctx *ctx);
+		struct iomap_read_folio_ctx *ctx, void *private);
 bool iomap_is_partially_uptodate(struct folio *, size_t from, size_t count);
 struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos, size_t len);
 bool iomap_release_folio(struct folio *folio, gfp_t gfp_flags);
@@ -595,7 +595,7 @@ static inline void iomap_bio_read_folio(struct folio *folio,
 		.cur_folio	= folio,
 	};
 
-	iomap_read_folio(ops, &ctx);
+	iomap_read_folio(ops, &ctx, NULL);
 }
 
 static inline void iomap_bio_readahead(struct readahead_control *rac,
@@ -606,7 +606,7 @@ static inline void iomap_bio_readahead(struct readahead_control *rac,
 		.rac		= rac,
 	};
 
-	iomap_readahead(ops, &ctx);
+	iomap_readahead(ops, &ctx, NULL);
 }
 #endif /* CONFIG_BLOCK */