This is a note to let you know that I've just added the patch titled
tracing: Add a vmalloc_sync_mappings() for safe measure
to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
tracing-add-a-vmalloc_sync_mappings-for-safe-measure.patch
and it can be found in the queue-4.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 11f5efc3ab66284f7aaacc926e9351d658e2577b Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
Date: Wed, 6 May 2020 10:36:18 -0400
Subject: tracing: Add a vmalloc_sync_mappings() for safe measure
From: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
commit 11f5efc3ab66284f7aaacc926e9351d658e2577b upstream.
x86_64 lazily maps in the vmalloc pages, and the way this works with per_cpu
areas can be complex, to say the least. Mappings may happen at boot up, and
if nothing synchronizes the page tables, those page mappings may not be
synced till they are used. This causes issues for anything that might touch
one of those mappings in the path of the page fault handler. When one of
those unmapped mappings is touched in the page fault handler, it will cause
another page fault, which in turn will cause a page fault, and leave us in
a loop of page faults.
Commit 763802b53a42 ("x86/mm: split vmalloc_sync_all()") split
vmalloc_sync_all() into vmalloc_sync_unmappings() and
vmalloc_sync_mappings(), as on system exit, it did not need to do a full
sync on x86_64 (although it still needed to be done on x86_32). By chance,
the vmalloc_sync_all() would synchronize the page mappings done at boot up
and prevent the per cpu area from being a problem for tracing in the page
fault handler. But when that synchronization in the exit of a task became a
nop, it caused the problem to appear.
Link: https://lore.kernel.org/r/20200429054857.66e8e333@xxxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Fixes: 737223fbca3b1 ("tracing: Consolidate buffer allocation code")
Reported-by: "Tzvetomir Stoyanov (VMware)" <tz.stoyanov@xxxxxxxxx>
Suggested-by: Joerg Roedel <jroedel@xxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/trace/trace.c | 13 +++++++++++++
1 file changed, 13 insertions(+)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6621,6 +6621,19 @@ static int allocate_trace_buffers(struct
*/
allocate_snapshot = false;
#endif
+
+ /*
+ * Because of some magic with the way alloc_percpu() works on
+ * x86_64, we need to synchronize the pgd of all the tables,
+ * otherwise the trace events that happen in x86_64 page fault
+ * handlers can't cope with accessing the chance that a
+ * alloc_percpu()'d memory might be touched in the page fault trace
+ * event. Oh, and we need to audit all other alloc_percpu() and vmalloc()
+ * calls in tracing, because something might get triggered within a
+ * page fault trace event!
+ */
+ vmalloc_sync_mappings();
+
return 0;
}
Patches currently in stable-queue which might be from rostedt@xxxxxxxxxxx are
queue-4.4/tracing-add-a-vmalloc_sync_mappings-for-safe-measure.patch
