linux-kernel - Re: [RFC PATCH 10/30] mm: enable page allocation tagging for __get_free_pages and alloc

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Date:   Wed, 31 Aug 2022 13:46:29 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Mel Gorman <mgorman@...e.de>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
        mhocko@...e.com, vbabka@...e.cz, hannes@...xchg.org,
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Subject: Re: [RFC PATCH 10/30] mm: enable page allocation tagging for
 __get_free_pages and alloc_pages

On Wed, Aug 31, 2022 at 11:11:03AM +0100, Mel Gorman wrote:
> On Tue, Aug 30, 2022 at 02:48:59PM -0700, Suren Baghdasaryan wrote:
> > Redefine alloc_pages, __get_free_pages to record allocations done by
> > these functions. Instrument deallocation hooks to record object freeing.
> > 
> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > +#ifdef CONFIG_PAGE_ALLOC_TAGGING
> > +
> >  #include <linux/alloc_tag.h>
> >  #include <linux/page_ext.h>
> >  
> > @@ -25,4 +27,37 @@ static inline void pgalloc_tag_dec(struct page *page, unsigned int order)
> >  		alloc_tag_sub(get_page_tag_ref(page), PAGE_SIZE << order);
> >  }
> >  
> > +/*
> > + * Redefinitions of the common page allocators/destructors
> > + */
> > +#define pgtag_alloc_pages(gfp, order)					\
> > +({									\
> > +	struct page *_page = _alloc_pages((gfp), (order));		\
> > +									\
> > +	if (_page)							\
> > +		alloc_tag_add(get_page_tag_ref(_page), PAGE_SIZE << (order));\
> > +	_page;								\
> > +})
> > +
> 
> Instead of renaming alloc_pages, why is the tagging not done in
> __alloc_pages()? At least __alloc_pages_bulk() is also missed. The branch
> can be guarded with IS_ENABLED.

It can't be in a function, it has to be in a wrapper macro.

alloc_tag_add() is a macro that defines a static struct in a special elf
section. That struct holds the allocation counters, and putting it in a special
elf section is how the code to list it in debugfs finds it.

Look at the dynamic debug code for prior precedence for this trick in the kernel
- that's how it makes pr_debug() calls dynamically controllable at runtime, from
debugfs. We're taking that method and turning it into a proper library.

Because all the counters are statically allocated, without even a pointer deref
to get to them in the allocation path (one pointer deref to get to them in the
deallocate path), that makes this _much, much_ cheaper than anything that could
be done with tracing - cheap enough that I expect many users will want to enable
it in production.