mm/filemap: fix page end in filemap_get_read_batch

commit 5956592ce337330cdff0399a6f8b6a5aea397a8e upstream.

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.

Link: https://lkml.kernel.org/r/20230208022400.28962-1-coolqyj@163.com
Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read")
Signed-off-by: Qian Yingjin <qian@ddn.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/mm/filemap.c b/mm/filemap.c
index 0834161..322aea7 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2569,18 +2569,19 @@ 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))