Re: [PATCH v1 5/8] mm, kasan: Stackdepot implementation. Enable stackdepot for SLAB

[Joonsoo Kim]

2016-02-17 3:37 GMT+09:00 Alexander Potapenko <glider@xxxxxxxxxx>:
> On Mon, Feb 1, 2016 at 3:55 AM, Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
>> On Thu, Jan 28, 2016 at 02:27:44PM +0100, Alexander Potapenko wrote:
>>> On Thu, Jan 28, 2016 at 1:51 PM, Alexander Potapenko <glider@xxxxxxxxxx> wrote:
>>> >
>>> > On Jan 28, 2016 8:40 AM, "Joonsoo Kim" <iamjoonsoo.kim@xxxxxxx> wrote:
>>> >>
>>> >> Hello,
>>> >>
>>> >> On Wed, Jan 27, 2016 at 07:25:10PM +0100, Alexander Potapenko wrote:
>>> >> > Stack depot will allow KASAN store allocation/deallocation stack traces
>>> >> > for memory chunks. The stack traces are stored in a hash table and
>>> >> > referenced by handles which reside in the kasan_alloc_meta and
>>> >> > kasan_free_meta structures in the allocated memory chunks.
>>> >>
>>> >> Looks really nice!
>>> >>
>>> >> Could it be more generalized to be used by other feature that need to
>>> >> store stack trace such as tracepoint or page owner?
>>> > Certainly yes, but see below.
>>> >
>>> >> If it could be, there is one more requirement.
>>> >> I understand the fact that entry is never removed from depot makes things
>>> >> very simpler, but, for general usecases, it's better to use reference
>>> >> count
>>> >> and allow to remove. Is it possible?
>>> > For our use case reference counting is not really necessary, and it would
>>> > introduce unwanted contention.
>>
>> Okay.
>>
>>> > There are two possible options, each having its advantages and drawbacks: we
>>> > can let the clients store the refcounters directly in their stacks (more
>>> > universal, but harder to use for the clients), or keep the counters in the
>>> > depot but add an API that does not change them (easier for the clients, but
>>> > potentially error-prone).
>>> > I'd say it's better to actually find at least one more user for the stack
>>> > depot in order to understand the requirements, and refactor the code after
>>> > that.
>>
>> I re-think the page owner case and it also may not need refcount.
>> For now, just moving this stuff to /lib would be helpful for other future user.
> I agree this code may need to be moved to /lib someday, but I wouldn't
> hurry with that.
> Right now it is quite KASAN-specific, and it's unclear yet whether
> anyone else is going to use it.
> I suggest we keep it in mm/kasan for now, and factor the common parts
> into /lib when the need arises.

Please consider it one more time. I really have a plan to use it on page owner,
because using page owner requires too many memory for stack trace and
it changes system behaviour a lot.

Page owner uses following structure to store stack trace.

struct page_ext {
        unsigned long flags;
#ifdef CONFIG_PAGE_OWNER
        unsigned int order;
        gfp_t gfp_mask;
        unsigned int nr_entries;
        int last_migrate_reason;
        unsigned long trace_entries[8];
#endif
};

Using stack depot in page owner would be straight forward if stack depot
is in /lib. It is possible to move it when needed but it requires moving
a file and it would not be desirable.

>> BTW, is there any performance number? I guess that it could affect
>> the performance.
> I've compared the performance of KASAN with SLAB allocator on a small
> synthetic benchmark in two modes: with stack depot enabled and with
> kasan_save_stack() unconditionally returning 0.
> In the former case 8% more time was spent in the kernel than in the latter case.
>
> If I am not mistaking, for SLUB allocator the bookkeeping (enabled
> with the slub_debug=UZ boot options) take only 1.5 time, so the
> difference is worth looking into (at least before we switch SLUB to
> stack depot).

Okay.