On 4/2/21 1:50 PM, Sergei Trofimovich wrote:
> On Thu, 1 Apr 2021 17:05:19 -0700
> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
>> On Thu, 1 Apr 2021 23:30:10 +0100 Sergei Trofimovich <slyfox@xxxxxxxxxx> wrote:
>>
>> > Before the change page_owner recursion was detected via fetching
>> > backtrace and inspecting it for current instruction pointer.
>> > It has a few problems:
>> > - it is slightly slow as it requires extra backtrace and a linear
>> > stack scan of the result
>> > - it is too late to check if backtrace fetching required memory
>> > allocation itself (ia64's unwinder requires it).
>> >
>> > To simplify recursion tracking let's use page_owner recursion depth
>> > as a counter in 'struct task_struct'.
>>
>> Seems like a better approach.
>>
>> > The change make page_owner=on work on ia64 bu avoiding infinite
>> > recursion in:
>> > kmalloc()
>> > -> __set_page_owner()
>> > -> save_stack()
>> > -> unwind() [ia64-specific]
>> > -> build_script()
>> > -> kmalloc()
>> > -> __set_page_owner() [we short-circuit here]
>> > -> save_stack()
>> > -> unwind() [recursion]
>> >
>> > ...
>> >
>> > --- a/include/linux/sched.h
>> > +++ b/include/linux/sched.h
>> > @@ -1371,6 +1371,15 @@ struct task_struct {
>> > struct llist_head kretprobe_instances;
>> > #endif
>> >
>> > +#ifdef CONFIG_PAGE_OWNER
>> > + /*
>> > + * Used by page_owner=on to detect recursion in page tracking.
>> > + * Is it fine to have non-atomic ops here if we ever access
>> > + * this variable via current->page_owner_depth?
>>
>> Yes, it is fine. This part of the comment can be removed.
>
> Cool! Will do.
>
>> > + */
>> > + unsigned int page_owner_depth;
>> > +#endif
>>
>> Adding to the task_struct has a cost. But I don't expect that
>> PAGE_OWNER is commonly used in prodction builds (correct?).
>
> Yeah, PAGE_OWNER should not be enabled for production kernels.
Note that it was converted to use a static key exactly so that it can be always
built in production kernels, and simply enabled on boot when needed. Our kernels
have it enabled.
> Not having extra memory overhead (or layout disruption) is a nice
> benefit though. I'll switch to "Unserialized, strictly 'current'" bitfield.
>
>> > --- a/init/init_task.c
>> > +++ b/init/init_task.c
>> > @@ -213,6 +213,9 @@ struct task_struct init_task
>> > #ifdef CONFIG_SECCOMP
>> > .seccomp = { .filter_count = ATOMIC_INIT(0) },
>> > #endif
>> > +#ifdef CONFIG_PAGE_OWNER
>> > + .page_owner_depth = 0,
>> > +#endif
>> > };
>> > EXPORT_SYMBOL(init_task);
>>
>> It will be initialized to zero by the compiler. We can omit this hunk
>> entirely.
>>
>> > --- a/mm/page_owner.c
>> > +++ b/mm/page_owner.c
>> > @@ -20,6 +20,16 @@
>> > */
>> > #define PAGE_OWNER_STACK_DEPTH (16)
>> >
>> > +/*
>> > + * How many reenters we allow to page_owner.
>> > + *
>> > + * Sometimes metadata allocation tracking requires more memory to be allocated:
>> > + * - when new stack trace is saved to stack depot
>> > + * - when backtrace itself is calculated (ia64)
>> > + * Instead of falling to infinite recursion give it a chance to recover.
>> > + */
>> > +#define PAGE_OWNER_MAX_RECURSION_DEPTH (1)
>>
>> So this is presently a boolean. Is there any expectation that
>> PAGE_OWNER_MAX_RECURSION_DEPTH will ever be greater than 1? If not, we
>> could use a single bit in the task_struct. Add it to the
>> "Unserialized, strictly 'current'" bitfields. Could make it a 2-bit field if we want
>> to permit PAGE_OWNER_MAX_RECURSION_DEPTH=larger.
>
> Let's settle on depth=1. depth>1 is not trivial for other reasons I don't
> completely understand.
That's fine, I don't think depth>1 would bring us much benefit anyway.
> Follow-up patch incoming.
>
