Re: [PATCH v10 05/14] mm: multi-gen LRU: groundwork

[Andrew Morton <akpm@linux-foundation.org>]

On Wed,  6 Apr 2022 21:15:17 -0600 Yu Zhao <yuzhao@google.com> wrote:

> Evictable pages are divided into multiple generations for each lruvec.
> The youngest generation number is stored in lrugen->max_seq for both
> anon and file types as they are aged on an equal footing. The oldest
> generation numbers are stored in lrugen->min_seq[] separately for anon
> and file types as clean file pages can be evicted regardless of swap
> constraints. These three variables are monotonically increasing.
> 
> ...
>
> +static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio, bool reclaiming)

There's a lot of function inlining here.  Fortunately the compiler will
ignore it all, because some of it looks wrong.  Please review (and
remeasure!).  If inlining is reqlly justified, use __always_inline, and
document the reasons for doing so.

> +{
> +	int gen;
> +	unsigned long old_flags, new_flags;
> +
> +	do {
> +		new_flags = old_flags = READ_ONCE(folio->flags);
> +		if (!(new_flags & LRU_GEN_MASK))
> +			return false;
> +
> +		VM_BUG_ON_FOLIO(folio_test_active(folio), folio);
> +		VM_BUG_ON_FOLIO(folio_test_unevictable(folio), folio);
> +
> +		gen = ((new_flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
> +
> +		new_flags &= ~LRU_GEN_MASK;
> +		/* for shrink_page_list() */
> +		if (reclaiming)
> +			new_flags &= ~(BIT(PG_referenced) | BIT(PG_reclaim));
> +		else if (lru_gen_is_active(lruvec, gen))
> +			new_flags |= BIT(PG_active);
> +	} while (cmpxchg(&folio->flags, old_flags, new_flags) != old_flags);

Clearly the cmpxchg loop is handling races against a concurrent
updater.  But it's unclear who that updater is, what are the dynamics
here and why the designer didn't use, say, spin_lock().  The way to
clarify such thigs is with code comments!

>  
> +#endif /* !__GENERATING_BOUNDS_H */
> +
> +/*
> + * Evictable pages are divided into multiple generations. The youngest and the
> + * oldest generation numbers, max_seq and min_seq, are monotonically increasing.
> + * They form a sliding window of a variable size [MIN_NR_GENS, MAX_NR_GENS]. An
> + * offset within MAX_NR_GENS, gen, indexes the LRU list of the corresponding

The "within MAX_NR_GENS, gen," text here is unclear?

> + * generation. The gen counter in folio->flags stores gen+1 while a page is on
> + * one of lrugen->lists[]. Otherwise it stores 0.
> + *
> + * A page is added to the youngest generation on faulting. The aging needs to
> + * check the accessed bit at least twice before handing this page over to the
> + * eviction. The first check takes care of the accessed bit set on the initial
> + * fault; the second check makes sure this page hasn't been used since then.
> + * This process, AKA second chance, requires a minimum of two generations,
> + * hence MIN_NR_GENS. And to maintain ABI compatibility with the active/inactive

Where is the ABI compatibility issue?  Is it in some way in which the
legacy LRU is presented to userspace?

> + * LRU, these two generations are considered active; the rest of generations, if
> + * they exist, are considered inactive. See lru_gen_is_active(). PG_active is
> + * always cleared while a page is on one of lrugen->lists[] so that the aging
> + * needs not to worry about it. And it's set again when a page considered active
> + * is isolated for non-reclaiming purposes, e.g., migration. See
> + * lru_gen_add_folio() and lru_gen_del_folio().
> + *
> + * MAX_NR_GENS is set to 4 so that the multi-gen LRU can support twice of the

"twice the number of"?
> + * categories of the active/inactive LRU when keeping track of accesses through
> + * page tables. It requires order_base_2(MAX_NR_GENS+1) bits in folio->flags.
> + */

Helpful comment, overall.

> 
> ...
>
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -909,6 +909,14 @@ config ANON_VMA_NAME
>  	  area from being merged with adjacent virtual memory areas due to the
>  	  difference in their name.
>  
> +config LRU_GEN
> +	bool "Multi-Gen LRU"
> +	depends on MMU
> +	# the following options can use up the spare bits in page flags
> +	depends on !MAXSMP && (64BIT || !SPARSEMEM || SPARSEMEM_VMEMMAP)
> +	help
> +	  A high performance LRU implementation to overcommit memory.
> +
>  source "mm/damon/Kconfig"

This is a problem.  I had to jump through hoops just to be able to
compile-test this.  Turns out I had to figure out how to disable
MAXSMP.

Can we please figure out a way to ensure that more testers are at least
compile testing this?  Allnoconfig, defconfig, allyesconfig, allmodconfig.

Also, I suggest that we actually make MGLRU the default while in linux-next.

>
> ...
>