[PATCH v2 3/3] zonefs: fix zonefs_iomap_begin() for reads

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

If a readahead) is issued to a sequential zone file with an offset
exactly equal to the current file size, the iomap type is set to
IOMAP_UNWRITTEN, which will prevent an IO, but the iomap length is
calculated as 0. This causes a WARN_ON() in iomap_iter():

[17309.548939] WARNING: CPU: 3 PID: 2137 at fs/iomap/iter.c:34 iomap_iter+0x9cf/0xe80
[...]
[17309.650907] RIP: 0010:iomap_iter+0x9cf/0xe80
[...]
[17309.754560] Call Trace:
[17309.757078]  <TASK>
[17309.759240]  ? lock_is_held_type+0xd8/0x130
[17309.763531]  iomap_readahead+0x1a8/0x870
[17309.767550]  ? iomap_read_folio+0x4c0/0x4c0
[17309.771817]  ? lockdep_hardirqs_on_prepare+0x400/0x400
[17309.778848]  ? lock_release+0x370/0x750
[17309.784462]  ? folio_add_lru+0x217/0x3f0
[17309.790220]  ? reacquire_held_locks+0x4e0/0x4e0
[17309.796543]  read_pages+0x17d/0xb60
[17309.801854]  ? folio_add_lru+0x238/0x3f0
[17309.807573]  ? readahead_expand+0x5f0/0x5f0
[17309.813554]  ? policy_node+0xb5/0x140
[17309.819018]  page_cache_ra_unbounded+0x27d/0x450
[17309.825439]  filemap_get_pages+0x500/0x1450
[17309.831444]  ? filemap_add_folio+0x140/0x140
[17309.837519]  ? lock_is_held_type+0xd8/0x130
[17309.843509]  filemap_read+0x28c/0x9f0
[17309.848953]  ? zonefs_file_read_iter+0x1ea/0x4d0 [zonefs]
[17309.856162]  ? trace_contention_end+0xd6/0x130
[17309.862416]  ? __mutex_lock+0x221/0x1480
[17309.868151]  ? zonefs_file_read_iter+0x166/0x4d0 [zonefs]
[17309.875364]  ? filemap_get_pages+0x1450/0x1450
[17309.881647]  ? __mutex_unlock_slowpath+0x15e/0x620
[17309.888248]  ? wait_for_completion_io_timeout+0x20/0x20
[17309.895231]  ? lock_is_held_type+0xd8/0x130
[17309.901115]  ? lock_is_held_type+0xd8/0x130
[17309.906934]  zonefs_file_read_iter+0x356/0x4d0 [zonefs]
[17309.913750]  new_sync_read+0x2d8/0x520
[17309.919035]  ? __x64_sys_lseek+0x1d0/0x1d0

Furthermore, this causes iomap_readahead() to loop forever as
iomap_readahead_iter() always returns 0, making no progress.

Fix this by treating reads after the file size as access to holes,
setting the iomap type to IOMAP_HOLE, the iomap addr to IOMAP_NULL_ADDR
and using the length argument as is for the iomap length. To simplify
the code with this change, zonefs_iomap_begin() is split into the read
variant, zonefs_read_iomap_begin() and zonefs_read_iomap_ops, and the
write variant, zonefs_write_iomap_begin() and zonefs_write_iomap_ops.

Reported-by: Jorgen Hansen <Jorgen.Hansen@wdc.com>
Fixes: 8dcc1a9d90c1 ("fs: New zonefs file system")
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
---
 fs/zonefs/super.c | 94 ++++++++++++++++++++++++++++++++---------------
 1 file changed, 64 insertions(+), 30 deletions(-)

diff --git a/fs/zonefs/super.c b/fs/zonefs/super.c
index 123464d2145a..053299758deb 100644
--- a/fs/zonefs/super.c
+++ b/fs/zonefs/super.c
@@ -110,15 +110,51 @@ static inline void zonefs_i_size_write(struct inode *inode, loff_t isize)
 	}
 }
 
-static int zonefs_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
-			      unsigned int flags, struct iomap *iomap,
-			      struct iomap *srcmap)
+static int zonefs_read_iomap_begin(struct inode *inode, loff_t offset,
+				   loff_t length, unsigned int flags,
+				   struct iomap *iomap, struct iomap *srcmap)
 {
 	struct zonefs_inode_info *zi = ZONEFS_I(inode);
 	struct super_block *sb = inode->i_sb;
 	loff_t isize;
 
-	/* All I/Os should always be within the file maximum size */
+	/*
+	 * All blocks are always mapped below EOF. If reading past EOF,
+	 * act as if there is a hole up to the file maximum size.
+	 */
+	mutex_lock(&zi->i_truncate_mutex);
+	iomap->bdev = inode->i_sb->s_bdev;
+	iomap->offset = ALIGN_DOWN(offset, sb->s_blocksize);
+	isize = i_size_read(inode);
+	if (iomap->offset >= isize) {
+		iomap->type = IOMAP_HOLE;
+		iomap->addr = IOMAP_NULL_ADDR;
+		iomap->length = length;
+	} else {
+		iomap->type = IOMAP_MAPPED;
+		iomap->addr = (zi->i_zsector << SECTOR_SHIFT) + iomap->offset;
+		iomap->length = isize - iomap->offset;
+	}
+	mutex_unlock(&zi->i_truncate_mutex);
+
+	trace_zonefs_iomap_begin(inode, iomap);
+
+	return 0;
+}
+
+static const struct iomap_ops zonefs_read_iomap_ops = {
+	.iomap_begin	= zonefs_read_iomap_begin,
+};
+
+static int zonefs_write_iomap_begin(struct inode *inode, loff_t offset,
+				    loff_t length, unsigned int flags,
+				    struct iomap *iomap, struct iomap *srcmap)
+{
+	struct zonefs_inode_info *zi = ZONEFS_I(inode);
+	struct super_block *sb = inode->i_sb;
+	loff_t isize;
+
+	/* All write I/Os should always be within the file maximum size */
 	if (WARN_ON_ONCE(offset + length > zi->i_max_size))
 		return -EIO;
 
@@ -128,7 +164,7 @@ static int zonefs_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 	 * operation.
 	 */
 	if (WARN_ON_ONCE(zi->i_ztype == ZONEFS_ZTYPE_SEQ &&
-			 (flags & IOMAP_WRITE) && !(flags & IOMAP_DIRECT)))
+			 !(flags & IOMAP_DIRECT)))
 		return -EIO;
 
 	/*
@@ -137,47 +173,44 @@ static int zonefs_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 	 * write pointer) and unwriten beyond.
 	 */
 	mutex_lock(&zi->i_truncate_mutex);
+	iomap->bdev = inode->i_sb->s_bdev;
+	iomap->offset = ALIGN_DOWN(offset, sb->s_blocksize);
+	iomap->addr = (zi->i_zsector << SECTOR_SHIFT) + iomap->offset;
 	isize = i_size_read(inode);
-	if (offset >= isize)
+	if (iomap->offset >= isize) {
 		iomap->type = IOMAP_UNWRITTEN;
-	else
+		iomap->length = zi->i_max_size - iomap->offset;
+	} else {
 		iomap->type = IOMAP_MAPPED;
-	if (flags & IOMAP_WRITE)
-		length = zi->i_max_size - offset;
-	else
-		length = min(length, isize - offset);
+		iomap->length = isize - iomap->offset;
+	}
 	mutex_unlock(&zi->i_truncate_mutex);
 
-	iomap->offset = ALIGN_DOWN(offset, sb->s_blocksize);
-	iomap->length = ALIGN(offset + length, sb->s_blocksize) - iomap->offset;
-	iomap->bdev = inode->i_sb->s_bdev;
-	iomap->addr = (zi->i_zsector << SECTOR_SHIFT) + iomap->offset;
-
 	trace_zonefs_iomap_begin(inode, iomap);
 
 	return 0;
 }
 
-static const struct iomap_ops zonefs_iomap_ops = {
-	.iomap_begin	= zonefs_iomap_begin,
+static const struct iomap_ops zonefs_write_iomap_ops = {
+	.iomap_begin	= zonefs_write_iomap_begin,
 };
 
 static int zonefs_read_folio(struct file *unused, struct folio *folio)
 {
-	return iomap_read_folio(folio, &zonefs_iomap_ops);
+	return iomap_read_folio(folio, &zonefs_read_iomap_ops);
 }
 
 static void zonefs_readahead(struct readahead_control *rac)
 {
-	iomap_readahead(rac, &zonefs_iomap_ops);
+	iomap_readahead(rac, &zonefs_read_iomap_ops);
 }
 
 /*
  * Map blocks for page writeback. This is used only on conventional zone files,
  * which implies that the page range can only be within the fixed inode size.
  */
-static int zonefs_map_blocks(struct iomap_writepage_ctx *wpc,
-			     struct inode *inode, loff_t offset)
+static int zonefs_write_map_blocks(struct iomap_writepage_ctx *wpc,
+				   struct inode *inode, loff_t offset)
 {
 	struct zonefs_inode_info *zi = ZONEFS_I(inode);
 
@@ -191,12 +224,12 @@ static int zonefs_map_blocks(struct iomap_writepage_ctx *wpc,
 	    offset < wpc->iomap.offset + wpc->iomap.length)
 		return 0;
 
-	return zonefs_iomap_begin(inode, offset, zi->i_max_size - offset,
-				  IOMAP_WRITE, &wpc->iomap, NULL);
+	return zonefs_write_iomap_begin(inode, offset, zi->i_max_size - offset,
+					IOMAP_WRITE, &wpc->iomap, NULL);
 }
 
 static const struct iomap_writeback_ops zonefs_writeback_ops = {
-	.map_blocks		= zonefs_map_blocks,
+	.map_blocks		= zonefs_write_map_blocks,
 };
 
 static int zonefs_writepage(struct page *page, struct writeback_control *wbc)
@@ -226,7 +259,8 @@ static int zonefs_swap_activate(struct swap_info_struct *sis,
 		return -EINVAL;
 	}
 
-	return iomap_swapfile_activate(sis, swap_file, span, &zonefs_iomap_ops);
+	return iomap_swapfile_activate(sis, swap_file, span,
+				       &zonefs_read_iomap_ops);
 }
 
 static const struct address_space_operations zonefs_file_aops = {
@@ -647,7 +681,7 @@ static vm_fault_t zonefs_filemap_page_mkwrite(struct vm_fault *vmf)
 
 	/* Serialize against truncates */
 	filemap_invalidate_lock_shared(inode->i_mapping);
-	ret = iomap_page_mkwrite(vmf, &zonefs_iomap_ops);
+	ret = iomap_page_mkwrite(vmf, &zonefs_write_iomap_ops);
 	filemap_invalidate_unlock_shared(inode->i_mapping);
 
 	sb_end_pagefault(inode->i_sb);
@@ -899,7 +933,7 @@ static ssize_t zonefs_file_dio_write(struct kiocb *iocb, struct iov_iter *from)
 	if (append)
 		ret = zonefs_file_dio_append(iocb, from);
 	else
-		ret = iomap_dio_rw(iocb, from, &zonefs_iomap_ops,
+		ret = iomap_dio_rw(iocb, from, &zonefs_write_iomap_ops,
 				   &zonefs_write_dio_ops, 0, NULL, 0);
 	if (zi->i_ztype == ZONEFS_ZTYPE_SEQ &&
 	    (ret > 0 || ret == -EIOCBQUEUED)) {
@@ -948,7 +982,7 @@ static ssize_t zonefs_file_buffered_write(struct kiocb *iocb,
 	if (ret <= 0)
 		goto inode_unlock;
 
-	ret = iomap_file_buffered_write(iocb, from, &zonefs_iomap_ops);
+	ret = iomap_file_buffered_write(iocb, from, &zonefs_write_iomap_ops);
 	if (ret > 0)
 		iocb->ki_pos += ret;
 	else if (ret == -EIO)
@@ -1041,7 +1075,7 @@ static ssize_t zonefs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 			goto inode_unlock;
 		}
 		file_accessed(iocb->ki_filp);
-		ret = iomap_dio_rw(iocb, to, &zonefs_iomap_ops,
+		ret = iomap_dio_rw(iocb, to, &zonefs_read_iomap_ops,
 				   &zonefs_read_dio_ops, 0, NULL, 0);
 	} else {
 		ret = generic_file_read_iter(iocb, to);
-- 
2.36.1