Date:	Wed, 4 May 2016 11:21:33 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>, mgorman@...hsingularity.net,
	Minchan Kim <minchan@...nel.org>,
	Alexander Potapenko <glider@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] mm/page_owner: use stackdepot to store stacktrace

On Wed 04-05-16 11:14:50, Joonsoo Kim wrote:
> On Tue, May 03, 2016 at 10:53:56AM +0200, Michal Hocko wrote:
> > On Tue 03-05-16 14:23:04, Joonsoo Kim wrote:
[...]
> > > Memory saving looks as following. (Boot 4GB memory system with page_owner)
> > > 
> > > 92274688 bytes -> 25165824 bytes
> > 
> > It is not clear to me whether this is after a fresh boot or some workload
> > which would grow the stack depot as well. What is a usual cap for the
> > memory consumption.
> 
> It is static allocation size after a fresh boot. I didn't add size of
> dynamic allocation memory so it could be larger a little. See below line.
> > 
> > > 72% reduction in static allocation size. Even if we should add up size of
> > > dynamic allocation memory, it would not that big because stacktrace is
> > > mostly duplicated.

This would be true only if most of the allocation stacks are basically
same after the boot which I am not really convinced is true. But you are
right that the number of sublicates will grow only a little. I was
interested about how much is that little ;)

> > > Note that implementation looks complex than someone would imagine because
> > > there is recursion issue. stackdepot uses page allocator and page_owner
> > > is called at page allocation. Using stackdepot in page_owner could re-call
> > > page allcator and then page_owner. That is a recursion.
> > 
> > This is rather fragile. How do we check there is no lock dependency
> > introduced later on - e.g. split_page called from a different
> > locking/reclaim context than alloc_pages? Would it be safer to
> 
> There is no callsite that calls set_page_owner() with
> __GFP_DIRECT_RECLAIM. So, there would be no lock/context dependency
> now.

I am confused now. prep_new_page is called with the gfp_mask of the
original request, no?

> split_page() doesn't call set_page_owner(). Instead, it calls
> split_page_owner() and just copies previous entry. Since it doesn't
> require any new stackdepot entry, it is safe in any context.

Ohh, you are right. I have missed patch 4
(http://lkml.kernel.org/r/1462252984-8524-5-git-send-email-iamjoonsoo.kim@lge.com)

> > use ~__GFP_DIRECT_RECLAIM for those stack allocations? Or do you think
> > there would be too many failed allocations? This alone wouldn't remove a
> > need for the recursion detection but it sounds less tricky.
> > 
> > > To detect and
> > > avoid it, whenever we obtain stacktrace, recursion is checked and
> > > page_owner is set to dummy information if found. Dummy information means
> > > that this page is allocated for page_owner feature itself
> > > (such as stackdepot) and it's understandable behavior for user.
> > > 
> > > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> > 
> > I like the idea in general I just wish this would be less subtle. Few
> > more comments below.
> > 
> > [...]
> > > -void __set_page_owner(struct page *page, unsigned int order, gfp_t gfp_mask)
> > > +static inline bool check_recursive_alloc(struct stack_trace *trace,
> > > +					unsigned long ip)
> > >  {
> > > -	struct page_ext *page_ext = lookup_page_ext(page);
> > > +	int i, count;
> > > +
> > > +	if (!trace->nr_entries)
> > > +		return false;
> > > +
> > > +	for (i = 0, count = 0; i < trace->nr_entries; i++) {
> > > +		if (trace->entries[i] == ip && ++count == 2)
> > > +			return true;
> > > +	}
> > 
> > This would deserve a comment I guess. Btw, don't we have a better and
> > more robust way to detect the recursion? Per task_struct flag or
> > something like that?
> 
> Okay. I will add a comment.
> 
> I already considered task_struct flag and I know that it is a better
> solution. But, I don't think that this debugging feature deserve to
> use such precious flag. This implementation isn't efficient but I
> think that it is at least robust.

I guess there are many holes in task_structs where a single bool would
comfortably fit in. But I do not consider this to be a large issue. It
is just the above looks quite ugly.

> > [...]
> > > +static noinline depot_stack_handle_t save_stack(gfp_t flags)
> > > +{
> > > +	unsigned long entries[PAGE_OWNER_STACK_DEPTH];
> > >  	struct stack_trace trace = {
> > >  		.nr_entries = 0,
> > > -		.max_entries = ARRAY_SIZE(page_ext->trace_entries),
> > > -		.entries = &page_ext->trace_entries[0],
> > > -		.skip = 3,
> > > +		.entries = entries,
> > > +		.max_entries = PAGE_OWNER_STACK_DEPTH,
> > > +		.skip = 0
> > >  	};
> > [...]
> > >  void __dump_page_owner(struct page *page)
> > >  {
> > >  	struct page_ext *page_ext = lookup_page_ext(page);
> > > +	unsigned long entries[PAGE_OWNER_STACK_DEPTH];
> > 
> > This is worrying because of the excessive stack consumption while we
> > might be in a deep call chain already. Can we preallocate a hash table
> > for few buffers when the feature is enabled? This would require locking
> > of course but chances are that contention wouldn't be that large.
> 
> Make sense but I'm not sure that excessive stack consumption would
> cause real problem. For example, if direct reclaim is triggered during
> allocation, it may go more deeper than this path.

Do we really consume 512B of stack during reclaim. That sounds more than
worrying to me.

> I'd like to postpone to handle this issue until stack breakage is
> reported due to this feature.

I dunno, but I would expect that a debugging feature wouldn't cause
problems like that. It is more than sad when you cannot debug your
issue just because of the stack consumption...

-- 
Michal Hocko
SUSE Labs

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