+ mm-filemap-fix-page-end-in-filemap_get_read_batch.patch added to mm-hotfixes-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/filemap: fix page end in filemap_get_read_batch
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-filemap-fix-page-end-in-filemap_get_read_batch.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-filemap-fix-page-end-in-filemap_get_read_batch.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Qian Yingjin <qian@ddn.com>
Subject: mm/filemap: fix page end in filemap_get_read_batch
Date: Wed, 8 Feb 2023 10:24:00 +0800

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>
---


--- a/mm/filemap.c~mm-filemap-fix-page-end-in-filemap_get_read_batch
+++ a/mm/filemap.c
@@ -2588,18 +2588,19 @@ static int filemap_get_pages(struct kioc
 	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))
_

Patches currently in -mm which might be from qian@ddn.com are

mm-filemap-fix-page-end-in-filemap_get_read_batch.patch