Commit 5956592c authored Feb 08, 2023 by Qian Yingjin Committed by Andrew Morton Feb 16, 2023

mm/filemap: fix page end in filemap_get_read_batch

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: cbd59c48 ("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>

parent ce4d9a1e

mm/filemap.c

+3 −2

Original line number	Diff line number	Diff line
		@@ -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))