+ test_xarray-add-tests-for-advanced-multi-index-use-fix.patch added to mm-unstable branch

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

The patch titled
     Subject: test_xarray: fix soft lockup for advanced-api tests
has been added to the -mm mm-unstable branch.  Its filename is
     test_xarray-add-tests-for-advanced-multi-index-use-fix.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/test_xarray-add-tests-for-advanced-multi-index-use-fix.patch

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

Before you just go and hit "reply", please:
   a) Consider who else should be cc'ed
   b) Prefer to cc a suitable mailing list as well
   c) Ideally: find the original patch on the mailing list and do a
      reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via the mm-everything
branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there every 2-3 working days

------------------------------------------------------
From: Luis Chamberlain <mcgrof@kernel.org>
Subject: test_xarray: fix soft lockup for advanced-api tests
Date: Fri, 16 Feb 2024 11:43:29 -0800

The new adanced API tests want to vet the xarray API is doing what it
promises by manually iterating over a set of possible indexes on its
own, and using a query operation which holds the RCU lock and then
releases it. So it is not using the helper loop options which xarray
provides on purpose. Any loop which iterates over 1 million entries
(which is possible with order 20, so emulating say a 4 GiB block size)
to just to rcu lock and unlock will eventually end up triggering a soft
lockup on systems which don't preempt, and have lock provin and RCU
prooving enabled.

xarray users already use XA_CHECK_SCHED for loops which may take a long
time, in our case we don't want to RCU unlock and lock as the caller
does that already, but rather just force a schedule every XA_CHECK_SCHED
iterations since the test is trying to not trust and rather test that
xarray is doing the right thing.


Link: https://lkml.kernel.org/r/20240216194329.840555-1-mcgrof@kernel.org
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lkml.kernel.org/r/202402071613.70f28243-lkp@intel.com
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Cc: Daniel Gomez <da.gomez@samsung.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 lib/test_xarray.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

--- a/lib/test_xarray.c~test_xarray-add-tests-for-advanced-multi-index-use-fix
+++ a/lib/test_xarray.c
@@ -728,6 +728,7 @@ static noinline void *test_get_entry(str
 {
 	XA_STATE(xas, xa, index);
 	void *p;
+	static unsigned int i = 0;
 
 	rcu_read_lock();
 repeat:
@@ -737,6 +738,17 @@ repeat:
 		goto repeat;
 	rcu_read_unlock();
 
+	/*
+	 * This is not part of the page cache, this selftest is pretty
+	 * aggressive and does not want to trust the xarray API but rather
+	 * test it, and for order 20 (4 GiB block size) we can loop over
+	 * over a million entries which can cause a soft lockup. Page cache
+	 * APIs won't be stupid, proper page cache APIs loop over the proper
+	 * order so when using a larger order we skip shared entries.
+	 */
+	if (++i % XA_CHECK_SCHED == 0)
+		schedule();
+
 	return p;
 }
 
_

Patches currently in -mm which might be from mcgrof@kernel.org are

test_xarray-add-tests-for-advanced-multi-index-use.patch
test_xarray-add-tests-for-advanced-multi-index-use-fix.patch