[PATCH] btrfs: skip call to generic_file_buffered_read if we don't need to

[Johannes Thumshirn <johannes.thumshirn@wdc.com>]

Since we switched to the iomap infrastructure in b5ff9f1a96e8f ("btrfs:
switch to iomap for direct IO") we're calling generic_file_buffered_read()
directly and not via generic_file_read_iter() anymore.

If the read could read everything there is no need to bother calling
generic_file_buffered_read(), like it is handled in
generic_file_read_iter().

If we call generic_file_buffered_read() in this case we can hit a
situation where we do an invalid readahead and cause this UBSAN splat:
johannes@redsun60:linux(btrfs-misc-next)$ kasan_symbolize.py < ubsan.txt
rapido1:/home/johannes/src/xfstests-dev# cat results/generic/091.dmesg
run fstests generic/091 at 2020-10-21 10:52:32
================================================================================
UBSAN: shift-out-of-bounds in ./include/linux/log2.h:57:13
shift exponent 64 is too large for 64-bit type 'long unsigned int'
CPU: 0 PID: 656 Comm: fsx Not tainted 5.9.0-rc7+ #821
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-0-gf21b5a4-rebuilt.opensuse.org 04/01/2014
Call Trace:
 __dump_stack lib/dump_stack.c:77
 dump_stack+0x57/0x70 lib/dump_stack.c:118
 ubsan_epilogue+0x5/0x40 lib/ubsan.c:148
 __ubsan_handle_shift_out_of_bounds.cold+0x61/0xe9 lib/ubsan.c:395
 __roundup_pow_of_two ./include/linux/log2.h:57
 get_init_ra_size mm/readahead.c:318
 ondemand_readahead.cold+0x16/0x2c mm/readahead.c:530
 generic_file_buffered_read+0x3ac/0x840 mm/filemap.c:2199
 call_read_iter ./include/linux/fs.h:1876
 new_sync_read+0x102/0x180 fs/read_write.c:415
 vfs_read+0x11c/0x1a0 fs/read_write.c:481
 ksys_read+0x4f/0xc0 fs/read_write.c:615
 do_syscall_64+0x33/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9 arch/x86/entry/entry_64.S:118
RIP: 0033:0x7fe87fee992e
Code: 0f 1f 40 00 48 8b 15 a1 96 00 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb ba 0f 1f 00 64 8b 04 25 18 00 00 00 85 c0 75 14 0f 05 <48> 3d 00 f0 ff ff 77 5a c3 66 0f 1f 84 00 00 00 00 00 48 83 ec 28
RSP: 002b:00007ffe01605278 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 000000000004f000 RCX: 00007fe87fee992e
RDX: 0000000000004000 RSI: 0000000001677000 RDI: 0000000000000003
RBP: 000000000004f000 R08: 0000000000004000 R09: 000000000004f000
R10: 0000000000053000 R11: 0000000000000246 R12: 0000000000004000
R13: 0000000000000000 R14: 000000000007a120 R15: 0000000000000000
================================================================================
BTRFS info (device nullb0): has skinny extents
BTRFS info (device nullb0): ZONED mode enabled, zone size 268435456 B
BTRFS info (device nullb0): enabling ssd optimizations

Fixes: b5ff9f1a96e8f ("btrfs: switch to iomap for direct IO")
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
---
 fs/btrfs/file.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 6f5ecba74f54..8e6def2c463d 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -3612,7 +3612,7 @@ static ssize_t btrfs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 		inode_lock_shared(inode);
 		ret = btrfs_direct_IO(iocb, to);
 		inode_unlock_shared(inode);
-		if (ret < 0)
+		if (ret < 0 || (ret > 0 && iov_iter_count(to) == 0))
 			return ret;
 	}
 
-- 
2.26.2