Re: [Patch] x86,mm: check freeze request in page fault handler

From: Cong Wang
Date: Tue Aug 12 2014 - 20:47:09 EST

On Tue, Aug 12, 2014 at 5:09 AM, Michal Hocko <mhocko@xxxxxxx> wrote:
> On Mon 11-08-14 17:53:55, Cong Wang wrote:
>> When a process triggers a page fault and kernel keeps
>> trying to retry the fault, there is no chance for this process
>> to be frozen, so the freeze request will always be pending.
> The retry cannot happen indefinitely, no?
> Besides that the patch is broken in at least 2 ways. You are not
> releasing mmap_sem and this will break memcg OOM killer handling.

Ah, it's my bad to miss the up_read().

> If a memcg is under OOM (because of hard limit) then try_charge
> calls mem_cgroup_oom which marks the current task with OOM
> information. Notably takes a reference to memcg->css. The charge fail
> will then gets up the pagefault stack until we get to mm_fault_error
> where you put the task into freezer and then returns without
> pagefault_out_of_memory which would handle memcg specific parts in
> mem_cgroup_oom_synchronize. If the task wakes up and the page fault
> retry succeeds (because some charges were released in the meantime) then
> you leak a reference to memcg->css.

OK, I thought skipping pagefault_out_of_memory() is safe, you are right here,
thanks for explanation.

> Besides that the whole change would need a better justification. Why
> other archs do not need this?

They probably need this as well, just that I only have x86 to care and test. :)

Does the following updated patch make any sense to you? If not, I will just
drop it.

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index a241946..8c0a7d8 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -14,6 +14,7 @@
#include <linux/hugetlb.h> /* hstate_index_to_shift */
#include <linux/prefetch.h> /* prefetchw */
#include <linux/context_tracking.h> /* exception_enter(), ... */
+#include <linux/freezer.h> /* try_to_freeze() */

#include <asm/traps.h> /* dotraplinkage, ... */
#include <asm/pgalloc.h> /* pgd_*(), ... */
@@ -904,6 +905,9 @@ mm_fault_error(struct pt_regs *regs, unsigned long
* oom-killed):
+ if (signal_pending(current))
+ try_to_freeze();
} else {
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