[PATCH] iomap: avoid redundant fault_in_iov_iter_readable() judgement when use larger chunks

[PATCH] iomap: avoid redundant fault_in_iov_iter_readable() judgement when use larger chunks

[Xu Yang]

Since commit (5d8edfb900d5 "iomap: Copy larger chunks from userspace"),
iomap will try to copy in larger chunks than PAGE_SIZE. However, if the
mapping doesn't support large folio, only one page of maximum 4KB will
be created and 4KB data will be writen to pagecache each time. Then,
next 4KB will be handled in next iteration.

If chunk is 2MB, total 512 pages need to be handled finally. During this
period, fault_in_iov_iter_readable() is called to check iov_iter readable
validity. Since only 4KB will be handled each time, below address space
will be checked over and over again:

pos         	len
-
start,    	2MB
start+4KB, 	2MB
start+8KB, 	2MB
...
start+2044KB 	2MB

Obviously the checking size is wrong since only 4KB will be handled each
time. So this will get a correct bytes before fault_in_iov_iter_readable()
to let iomap work well in non-large folio case.

Fixes: 5d8edfb900d5 ("iomap: Copy larger chunks from userspace")
Cc: stable@vger.kernel.org
Signed-off-by: Xu Yang <xu.yang_2@nxp.com>
---
 fs/iomap/buffered-io.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 41c8f0c68ef5..51ca31cd94ae 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -925,6 +925,9 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
 		if (bytes > length)
 			bytes = length;
 
+		if (!mapping_large_folio_support(iter->inode->i_mapping))
+			bytes = min_t(size_t, bytes, PAGE_SIZE - offset_in_page(pos));
+
 		/*
 		 * Bring in the user page that we'll copy from _first_.
 		 * Otherwise there's a nasty deadlock on copying from the
-- 
2.34.1