linux-kernel - Re: [PATCH 2/3] mm: add find_alloc_contig

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Date:   Sun, 22 Apr 2018 18:09:43 -0600
From:   Michal Hocko <mhocko@...nel.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org,
        Reinette Chatre <reinette.chatre@...el.com>,
        Christopher Lameter <cl@...ux.com>,
        Guy Shattah <sguy@...lanox.com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        Michal Nazarewicz <mina86@...a86.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Nellans <dnellans@...dia.com>,
        Laura Abbott <labbott@...hat.com>, Pavel Machek <pavel@....cz>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/3] mm: add find_alloc_contig_pages() interface

On Mon 16-04-18 19:09:14, Mike Kravetz wrote:
[...]
> @@ -2010,9 +2011,13 @@ static __always_inline struct page *__rmqueue_cma_fallback(struct zone *zone,
>  {
>  	return __rmqueue_smallest(zone, order, MIGRATE_CMA);
>  }
> +#define contig_alloc_migratetype_ok(migratetype) \
> +	((migratetype) == MIGRATE_CMA || (migratetype) == MIGRATE_MOVABLE)
>  #else
>  static inline struct page *__rmqueue_cma_fallback(struct zone *zone,
>  					unsigned int order) { return NULL; }
> +#define contig_alloc_migratetype_ok(migratetype) \
> +	((migratetype) == MIGRATE_MOVABLE)
>  #endif
>  
>  /*
> @@ -7822,6 +7827,9 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  	};
>  	INIT_LIST_HEAD(&cc.migratepages);
>  
> +	if (!contig_alloc_migratetype_ok(migratetype))
> +		return -EINVAL;
> +
>
>  	/*
>  	 * What we do here is we mark all pageblocks in range as
>  	 * MIGRATE_ISOLATE.  Because pageblock and max order pages may
> @@ -7912,8 +7920,9 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  
>  	/* Make sure the range is really isolated. */
>  	if (test_pages_isolated(outer_start, end, false)) {
> -		pr_info_ratelimited("%s: [%lx, %lx) PFNs busy\n",
> -			__func__, outer_start, end);
> +		if (!(migratetype == MIGRATE_MOVABLE)) /* only print for CMA */
> +			pr_info_ratelimited("%s: [%lx, %lx) PFNs busy\n",
> +				__func__, outer_start, end);
>  		ret = -EBUSY;
>  		goto done;
>  	}

This probably belongs to a separate patch. I would be tempted to say
that we should get rid of this migratetype thingy altogether. I confess
I have forgot everything about why this is required actually but it is
ugly as hell. Not your fault of course.

> @@ -7949,6 +7958,82 @@ void free_contig_range(unsigned long pfn, unsigned long nr_pages)
>  	}
>  	WARN(count != 0, "%ld pages are still in use!\n", count);
>  }
> +
> +static bool contig_pfn_range_valid(struct zone *z, unsigned long start_pfn,
> +					unsigned long nr_pages)
> +{
> +	unsigned long i, end_pfn = start_pfn + nr_pages;
> +	struct page *page;
> +
> +	for (i = start_pfn; i < end_pfn; i++) {
> +		if (!pfn_valid(i))
> +			return false;
> +
> +		page = pfn_to_page(i);

It believe we want pfn_to_online_page here. The old giga pages code is
buggy in that regard but nothing really critical because the
alloc_contig_range will notice that.

Also do we want to check other usual suspects? E.g. PageReserved? And
generally migrateable pages if page count > 0. Or do we want to leave
everything to the alloc_contig_range?

> +
> +		if (page_zone(page) != z)
> +			return false;
> +
> +	}
> +
> +	return true;
> +}
> +
> +/**
> + * find_alloc_contig_pages() -- attempt to find and allocate a contiguous
> + *				range of pages
> + * @order:	number of pages
> + * @gfp:	gfp mask used to limit search as well as during compaction
> + * @nid:	target node
> + * @nodemask:	mask of other possible nodes
> + *
> + * Pages can be freed with a call to free_contig_pages(), or by manually
> + * calling __free_page() for each page allocated.
> + *
> + * Return: pointer to 'order' pages on success, or NULL if not successful.
> + */
> +struct page *find_alloc_contig_pages(unsigned int order, gfp_t gfp,
> +					int nid, nodemask_t *nodemask)

Vlastimil asked about this but I would even say that we do not want to
make this order based. Why would we want to restrict the api to 2^order
sizes in the first place? What if somebody wants to allocate 123 pages?

> +{
> +	unsigned long pfn, nr_pages, flags;
> +	struct page *ret_page = NULL;
> +	struct zonelist *zonelist;
> +	struct zoneref *z;
> +	struct zone *zone;
> +	int rc;
> +
> +	nr_pages = 1 << order;
> +	zonelist = node_zonelist(nid, gfp);
> +	for_each_zone_zonelist_nodemask(zone, z, zonelist, gfp_zone(gfp),
> +					nodemask) {
> +		spin_lock_irqsave(&zone->lock, flags);
> +		pfn = ALIGN(zone->zone_start_pfn, nr_pages);
> +		while (zone_spans_pfn(zone, pfn + nr_pages - 1)) {
> +			if (contig_pfn_range_valid(zone, pfn, nr_pages)) {
> +				spin_unlock_irqrestore(&zone->lock, flags);

I know that the giga page allocation does use the zone lock but why? I
suspect it wants to stabilize zone_start_pfn but zone lock doesn't do
that.

> +
> +				rc = alloc_contig_range(pfn, pfn + nr_pages,
> +							MIGRATE_MOVABLE, gfp);
> +				if (!rc) {
> +					ret_page = pfn_to_page(pfn);
> +					return ret_page;
> +				}
> +				spin_lock_irqsave(&zone->lock, flags);
> +			}
> +			pfn += nr_pages;
> +		}
> +		spin_unlock_irqrestore(&zone->lock, flags);
> +	}

Other than that this API looks much saner than alloc_contig_range. We
still need to sort out some details (e.g. alignment) but it should be an
improvement.

Thanks!
-- 
Michal Hocko
SUSE Labs