File 0024-LU-16019-llite-fully-disable-readahead-in-kernel-I-O.patch of Package lustre_2_15

From ece396efd0ae71af00eca53226be4c82ca44ca52 Mon Sep 17 00:00:00 2001
From: Qian Yingjin <qian@ddn.com>
Date: Mon, 15 Aug 2022 11:15:25 -0700
Subject: [PATCH 24/30] LU-16019 llite: fully disable readahead in kernel I/O
 path

In the new kernel (rhel9 or ubuntu 2204), the readahead path may
be out of the control of Lustre CLIO engine:

generic_file_read_iter()
  ->filemap_read()
    ->filemap_get_pages()
      ->page_cache_sync_readahead()
        ->page_cache_sync_ra()

void page_cache_sync_ra()
{
	if (!ractl->ra->ra_pages || blk_cgroup_congested()) {
		if (!ractl->file)
			return;
		req_count = 1;
		do_forced_ra = true;
	}

	/* be dumb */
	if (do_forced_ra) {
		force_page_cache_ra(ractl, req_count);
		return;
	}
	...
}

From the kernel readahead code, even if read-ahead is disabled
(via @ra_pages == 0), it still issues this request as read-ahead
as we will need it to satisfy the requested range. The forced
read-ahead will do the right thing and limit the read to just
the requested range, which we will set to 1 page for this case.

Thus it can not totally avoid the read-ahead in the kernel I/O
path only by setting @ra_pages with 0.
To fully disable the read-ahead in the Linux kernel I/O path, we
still need to set @io_pages to 0, it will set I/O range to 0 in
@force_page_cache_ra():
void force_page_cache_ra()
{
	...
	max_pages = = max_t(unsigned long, bdi->io_pages,
			    ra->ra_pages);
	nr_to_read = min_t(unsigned long, nr_to_read, max_pages);
	while (nr_to_read) {
		...
	}
	...
}

After set bdi->io_pages with 0, it can pass the sanity/101j.

Lustre-change: https://review.whamcloud.com/47993
Lustre-commit: f0cf7fd3cccb2313fa94a307cf862afba256b8d8

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I859a6404abb9116d9acfa03de91e61d3536d3554
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
---
 lustre/autoconf/lustre-core.m4 | 22 ++++++++++++++++++++++
 lustre/llite/llite_lib.c       |  3 +++
 2 files changed, 25 insertions(+)

diff --git a/lustre/autoconf/lustre-core.m4 b/lustre/autoconf/lustre-core.m4
index cdb45cc133e5..49bc2a96376f 100644
--- a/lustre/autoconf/lustre-core.m4
+++ b/lustre/autoconf/lustre-core.m4
@@ -1867,6 +1867,27 @@ posix_acl_update_mode, [
 ])
 ]) # LC_POSIX_ACL_UPDATE_MODE
 
+#
+# LC_HAVE_BDI_IO_PAGES
+#
+# Kernel version 4.9 commit 9491ae4aade6814afcfa67f4eb3e3342c2b39750
+# mm: don't cap request size based on read-ahead setting
+# This patch introduces a bdi hint, io_pages.
+#
+AC_DEFUN([LC_HAVE_BDI_IO_PAGES], [
+LB_CHECK_COMPILE([if 'struct backing_dev_info' has 'io_pages' field],
+bdi_has_io_pages, [
+	#include <linux/backing-dev.h>
+],[
+	struct backing_dev_info info;
+
+	info.io_pages = 0;
+],[
+	AC_DEFINE(HAVE_BDI_IO_PAGES, 1,
+		[backing_dev_info has io_pages])
+])
+]) # LC_HAVE_BDI_IO_PAGES
+
 #
 # LC_IOP_GENERIC_READLINK
 #
@@ -2755,6 +2776,7 @@ AC_DEFUN([LC_PROG_LINUX], [
 	LC_GROUP_INFO_GID
 	LC_VFS_SETXATTR
 	LC_POSIX_ACL_UPDATE_MODE
+	LC_HAVE_BDI_IO_PAGES
 
 	# 4.10
 	LC_IOP_GENERIC_READLINK
diff --git a/lustre/llite/llite_lib.c b/lustre/llite/llite_lib.c
index 07649684f538..74d2291f5f26 100644
--- a/lustre/llite/llite_lib.c
+++ b/lustre/llite/llite_lib.c
@@ -1347,6 +1347,9 @@ int ll_fill_super(struct super_block *sb)
 
 	/* disable kernel readahead */
 	sb->s_bdi->ra_pages = 0;
+#ifdef HAVE_BDI_IO_PAGES
+	sb->s_bdi->io_pages = 0;
+#endif
 
 	/* Call ll_debugfs_register_super() before lustre_process_log()
 	 * so that "llite.*.*" params can be processed correctly.
-- 
2.38.1