Re: [PATCHv8 02/14] mm: Add support for unaccepted memory

[Vlastimil Babka]

On 12/24/22 17:46, Kirill A. Shutemov wrote:
> On Fri, Dec 09, 2022 at 11:23:50PM +0100, Vlastimil Babka wrote:
>> On 12/9/22 20:26, Kirill A. Shutemov wrote:
>> >> >  #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
>> >> >  			/*
>> >> >  			 * Watermark failed for this zone, but see if we can
>> >> > @@ -4299,6 +4411,9 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
>> >> >  
>> >> >  			return page;
>> >> >  		} else {
>> >> > +			if (try_to_accept_memory(zone))
>> >> > +				goto try_this_zone;
>> >> 
>> >> On the other hand, here we failed the full rmqueue(), including the
>> >> potentially fragmenting fallbacks, so I'm worried that before we finally
>> >> fail all of that and resort to accepting more memory, we already fragmented
>> >> the already accepted memory, more than necessary.
>> > 
>> > I'm not sure I follow. We accept memory in pageblock chunks. Do we want to
>> > allocate from a free pageblock if we have other memory to tap from? It
>> > doesn't make sense to me.
>> 
>> The fragmentation avoidance based on migratetype does work with pageblock
>> granularity, so yeah, if you accept a single pageblock worth of memory and
>> then (through __rmqueue_fallback()) end up serving both movable and
>> unmovable allocations from it, the whole fragmentation avoidance mechanism
>> is defeated and you end up with unmovable allocations (e.g. page tables)
>> scattered over many pageblocks and inability to allocate any huge pages.
>> 
>> >> So one way to prevent would be to move the acceptance into rmqueue() to
>> >> happen before __rmqueue_fallback(), which I originally had in mind and maybe
>> >> suggested that previously.
>> > 
>> > I guess it should be pretty straight forward to fail __rmqueue_fallback()
>> > if there's non-empty unaccepted_pages list and steer to
>> > try_to_accept_memory() this way.
>> 
>> That could be a way indeed. We do have ALLOC_NOFRAGMENT which could be
>> possible to employ here.
>> But maybe the zone_watermark_fast() modification would be simpler yet
>> sufficient. It makes sense to me that we'd try to keep a high watermark
>> worth of pre-accepted memory. zone_watermark_fast() would fail at low
>> watermark, so we could try accepting (high-low) at a time instead of single
>> pageblock.
> 
> Looks like we already have __zone_watermark_unusable_free() that seems
> match use-case rather closely. We only need switch unaccepted memory to
> per-zone accounting.

Could work. I'd still suggest also making try_to_accept_memory() to accept
up to high watermark, not a single pageblock.
> The fixup below suppose to do the trick, but I'm not sure how to test
> fragmentation avoidance properly.
> 
> Any suggestions?

Haven't done that for years, maybe Mel knows better. But from what I
remember, I'd compare /proc/pagetypeinfo with and without memory accepting,
and collect the mm_page_alloc_extfrag tracepoint. If there are more of these
events happening, it's bad. Ideally with a workload that stresses both
userspace (movable) allocations and kernel allocations. Again, Mel might
have suggestions for a mmtest?

> 
> diff --git a/drivers/base/node.c b/drivers/base/node.c
> index ca6f0590be21..1bd2d245edee 100644
> --- a/drivers/base/node.c
> +++ b/drivers/base/node.c
> @@ -483,7 +483,7 @@ static ssize_t node_read_meminfo(struct device *dev,
>  #endif
>  #ifdef CONFIG_UNACCEPTED_MEMORY
>  			     ,
> -			     nid, K(node_page_state(pgdat, NR_UNACCEPTED))
> +			     nid, K(sum_zone_node_page_state(nid, NR_UNACCEPTED))
>  #endif
>  			    );
>  	len += hugetlb_report_node_meminfo(buf, len, nid);
> diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
> index 789b77c7b6df..e9c05b4c457c 100644
> --- a/fs/proc/meminfo.c
> +++ b/fs/proc/meminfo.c
> @@ -157,7 +157,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
>  
>  #ifdef CONFIG_UNACCEPTED_MEMORY
>  	show_val_kb(m, "Unaccepted:     ",
> -		    global_node_page_state(NR_UNACCEPTED));
> +		    global_zone_page_state(NR_UNACCEPTED));
>  #endif
>  
>  	hugetlb_report_meminfo(m);
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 9c762e8175fc..8b5800cd4424 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -152,6 +152,9 @@ enum zone_stat_item {
>  	NR_ZSPAGES,		/* allocated in zsmalloc */
>  #endif
>  	NR_FREE_CMA_PAGES,
> +#ifdef CONFIG_UNACCEPTED_MEMORY
> +	NR_UNACCEPTED,
> +#endif
>  	NR_VM_ZONE_STAT_ITEMS };
>  
>  enum node_stat_item {
> @@ -198,9 +201,6 @@ enum node_stat_item {
>  	NR_FOLL_PIN_ACQUIRED,	/* via: pin_user_page(), gup flag: FOLL_PIN */
>  	NR_FOLL_PIN_RELEASED,	/* pages returned via unpin_user_page() */
>  	NR_KERNEL_STACK_KB,	/* measured in KiB */
> -#ifdef CONFIG_UNACCEPTED_MEMORY
> -	NR_UNACCEPTED,
> -#endif
>  #if IS_ENABLED(CONFIG_SHADOW_CALL_STACK)
>  	NR_KERNEL_SCS_KB,	/* measured in KiB */
>  #endif
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e80e8d398863..404b267332a9 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1779,7 +1779,7 @@ static bool try_to_accept_memory(struct zone *zone)
>  
>  	migratetype = get_pfnblock_migratetype(page, page_to_pfn(page));
>  	__mod_zone_freepage_state(zone, -1 << order, migratetype);
> -	__mod_node_page_state(page_pgdat(page), NR_UNACCEPTED, -1 << order);
> +	__mod_zone_page_state(zone, NR_UNACCEPTED, -1 << order);
>  	spin_unlock_irqrestore(&zone->lock, flags);
>  
>  	if (last)
> @@ -1808,7 +1808,7 @@ static void __free_unaccepted(struct page *page, unsigned int order)
>  	migratetype = get_pfnblock_migratetype(page, page_to_pfn(page));
>  	list_add_tail(&page->lru, &zone->unaccepted_pages);
>  	__mod_zone_freepage_state(zone, 1 << order, migratetype);
> -	__mod_node_page_state(page_pgdat(page), NR_UNACCEPTED, 1 << order);
> +	__mod_zone_page_state(zone, NR_UNACCEPTED, 1 << order);
>  	spin_unlock_irqrestore(&zone->lock, flags);
>  
>  	if (first)
> @@ -4074,6 +4074,9 @@ static inline long __zone_watermark_unusable_free(struct zone *z,
>  	if (!(alloc_flags & ALLOC_CMA))
>  		unusable_free += zone_page_state(z, NR_FREE_CMA_PAGES);
>  #endif
> +#ifdef CONFIG_UNACCEPTED_MEMORY
> +	unusable_free += zone_page_state(z, NR_UNACCEPTED);
> +#endif
>  
>  	return unusable_free;
>  }