From: Qian Yingjin <qian@xxxxxxx>
I was running traces of the read code against an RAID storage
system to understand why read requests were being misaligned
against the underlying RAID strips. I found that the page end
offset calculation in filemap_get_read_batch() was off by one.
When a read is submitted with end offset 1048575, then it
calculates the end page for read of 256 when it should be 255.
"last_index" is the index of the page beyond the end of the read
and it should be skipped when get a batch of pages for read in
@filemap_get_read_batch().
The below simple patch fixes the problem. This code was introduced
in kernel 5.12.
Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read")
Signed-off-by: Qian Yingjin <qian@xxxxxxx>
---
mm/filemap.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/mm/filemap.c b/mm/filemap.c
index c4d4ace9cc70..0e20a8d6dd93 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2588,18 +2588,19 @@ static int filemap_get_pages(struct kiocb *iocb, struct iov_iter *iter,
struct folio *folio;
int err = 0;
+ /* "last_index" is the index of the page beyond the end of the read */
last_index = DIV_ROUND_UP(iocb->ki_pos + iter->count, PAGE_SIZE);
retry:
if (fatal_signal_pending(current))
return -EINTR;
- filemap_get_read_batch(mapping, index, last_index, fbatch);
+ filemap_get_read_batch(mapping, index, last_index - 1, fbatch);
if (!folio_batch_count(fbatch)) {
if (iocb->ki_flags & IOCB_NOIO)
return -EAGAIN;
page_cache_sync_readahead(mapping, ra, filp, index,
last_index - index);
- filemap_get_read_batch(mapping, index, last_index, fbatch);
+ filemap_get_read_batch(mapping, index, last_index - 1, fbatch);
}
if (!folio_batch_count(fbatch)) {
if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_WAITQ))
--
2.34.1
