Date:   Wed, 2 Dec 2020 10:14:41 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        hyesoo.yu@...sung.com, willy@...radead.org, iamjoonsoo.kim@....com,
        vbabka@...e.cz, surenb@...gle.com, pullip.cho@...sung.com,
        joaodias@...gle.com, hridya@...gle.com, sumit.semwal@...aro.org,
        john.stultz@...aro.org, Brian.Starkey@....com,
        linux-media@...r.kernel.org, devicetree@...r.kernel.org,
        robh@...nel.org, christian.koenig@....com,
        linaro-mm-sig@...ts.linaro.org, Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH v2 2/4] mm: introduce cma_alloc_bulk API

On 01.12.20 18:51, Minchan Kim wrote:
> There is a need for special HW to require bulk allocation of
> high-order pages. For example, 4800 * order-4 pages, which
> would be minimum, sometimes, it requires more.
> 
> To meet the requirement, a option reserves 300M CMA area and
> requests the whole 300M contiguous memory. However, it doesn't
> work if even one of those pages in the range is long-term pinned
> directly or indirectly. The other option is to ask higher-order

My latest knowledge is that pages in the CMA area are never long term
pinned.

https://lore.kernel.org/lkml/20201123090129.GD27488@dhcp22.suse.cz/

"gup already tries to deal with long term pins on CMA regions and migrate
to a non CMA region. Have a look at __gup_longterm_locked."

We should rather identify ways how that is still possible and get rid of
them.


Now, short-term pinnings and PCP are other issues where
alloc_contig_range() could be improved (e.g., in contrast to a FAST
mode, a HARD mode which temporarily disables the PCP, ...).

> size (e.g., 2M) than requested order(64K) repeatedly until driver
> could gather necessary amount of memory. Basically, this approach
> makes the allocation very slow due to cma_alloc's function
> slowness and it could be stuck on one of the pageblocks if it
> encounters unmigratable page.
> 
> To solve the issue, this patch introduces cma_alloc_bulk.
> 
> 	int cma_alloc_bulk(struct cma *cma, unsigned int align,
> 		bool fast, unsigned int order, size_t nr_requests,
> 		struct page **page_array, size_t *nr_allocated);
> 
> Most parameters are same with cma_alloc but it additionally passes
> vector array to store allocated memory. What's different with cma_alloc
> is it will skip pageblocks without waiting/stopping if it has unmovable
> page so that API continues to scan other pageblocks to find requested
> order page.
> 
> cma_alloc_bulk is best effort approach in that it skips some pageblocks
> if they have unmovable pages unlike cma_alloc. It doesn't need to be
> perfect from the beginning at the cost of performance. Thus, the API
> takes "bool fast parameter" which is propagated into alloc_contig_range to
> avoid significat overhead functions to inrecase CMA allocation success
> ratio(e.g., migration retrial, PCP, LRU draining per pageblock)
> at the cost of less allocation success ratio. If the caller couldn't
> allocate enough, they could call it with "false" to increase success ratio
> if they are okay to expense the overhead for the success ratio.

Just so I understand what the idea is:

alloc_contig_range() sometimes fails on CMA regions when trying to
allocate big chunks (e.g., 300M). Instead of tackling that issue, you
rather allocate plenty of small chunks, and make these small allocations
fail faster/ make the allocations less reliable. Correct?

I don't really have a strong opinion on that. Giving up fast rather than
trying for longer sounds like a useful thing to have - but I wonder if
it's strictly necessary for the use case you describe.

I'd like to hear Michals opinion on that.

> 
> Signed-off-by: Minchan Kim <minchan@...nel.org>
> ---
>  include/linux/cma.h |   5 ++
>  include/linux/gfp.h |   2 +
>  mm/cma.c            | 126 ++++++++++++++++++++++++++++++++++++++++++--
>  mm/page_alloc.c     |  19 ++++---
>  4 files changed, 140 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/cma.h b/include/linux/cma.h
> index 217999c8a762..7375d3131804 100644
> --- a/include/linux/cma.h
> +++ b/include/linux/cma.h
> @@ -46,6 +46,11 @@ extern int cma_init_reserved_mem(phys_addr_t base, phys_addr_t size,
>  					struct cma **res_cma);
>  extern struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
>  			      bool no_warn);
> +
> +extern int cma_alloc_bulk(struct cma *cma, unsigned int align, bool fast,
> +			unsigned int order, size_t nr_requests,
> +			struct page **page_array, size_t *nr_allocated);
> +
>  extern bool cma_release(struct cma *cma, const struct page *pages, unsigned int count);
>  
>  extern int cma_for_each_area(int (*it)(struct cma *cma, void *data), void *data);
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index ad5872699692..75bfb673d75b 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -627,6 +627,8 @@ static inline bool pm_suspended_storage(void)
>  enum alloc_contig_mode {
>  	/* try several ways to increase success ratio of memory allocation */
>  	ALLOC_CONTIG_NORMAL,
> +	/* avoid costly functions to make the call fast */
> +	ALLOC_CONTIG_FAST,
>  };
>  
>  /* The below functions must be run on a range from a single zone. */
> diff --git a/mm/cma.c b/mm/cma.c
> index 8010c1ba04b0..4459045fa717 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -32,6 +32,7 @@
>  #include <linux/highmem.h>
>  #include <linux/io.h>
>  #include <linux/kmemleak.h>
> +#include <linux/swap.h>
>  #include <trace/events/cma.h>
>  
>  #include "cma.h"
> @@ -397,6 +398,14 @@ static void cma_debug_show_areas(struct cma *cma)
>  static inline void cma_debug_show_areas(struct cma *cma) { }
>  #endif
>  
> +static void reset_page_kasan_tag(struct page *page, int count)
> +{
> +	int i;
> +
> +	for (i = 0; i < count; i++)
> +		page_kasan_tag_reset(page + i);
> +}
> +
>  /**
>   * cma_alloc() - allocate pages from contiguous area
>   * @cma:   Contiguous memory region for which the allocation is performed.
> @@ -414,7 +423,6 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
>  	unsigned long pfn = -1;
>  	unsigned long start = 0;
>  	unsigned long bitmap_maxno, bitmap_no, bitmap_count;
> -	size_t i;
>  	struct page *page = NULL;
>  	int ret = -ENOMEM;
>  
> @@ -479,10 +487,8 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
>  	 * blocks being marked with different tags. Reset the tags to ignore
>  	 * those page blocks.
>  	 */
> -	if (page) {
> -		for (i = 0; i < count; i++)
> -			page_kasan_tag_reset(page + i);
> -	}
> +	if (page)
> +		reset_page_kasan_tag(page, count);
>  
>  	if (ret && !no_warn) {
>  		pr_err("%s: alloc failed, req-size: %zu pages, ret: %d\n",
> @@ -494,6 +500,116 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
>  	return page;
>  }
>  
> +/*
> + * cma_alloc_bulk() - allocate high order bulk pages from contiguous area with
> + * 		best effort. It will usually be used for private @cma
> + *
> + * @cma:	contiguous memory region for which the allocation is performed.
> + * @align:	requested alignment of pages (in PAGE_SIZE order).
> + * @fast:	will skip costly opeartions if it's true.
> + * @order:	requested page order
> + * @nr_requests: the number of 2^order pages requested to be allocated as input,
> + * @page_array:	page_array pointer to store allocated pages (must have space
> + *		for at least nr_requests)
> + * @nr_allocated: the number of 2^order pages allocated as output
> + *
> + * This function tries to allocate up to @nr_requests @order pages on specific
> + * contiguous memory area. If @fast has true, it will avoid costly functions
> + * to increase allocation success ratio so it will be faster but might return
> + * less than requested number of pages. User could retry it with true if it is
> + * needed.
> + *
> + * Return: it will return 0 only if all pages requested by @nr_requestsed are
> + * allocated. Otherwise, it returns negative error code.
> + *
> + * Note: Regardless of success/failure, user should check @nr_allocated to see
> + * how many @order pages are allocated and free those pages when they are not
> + * needed.
> + */
> +int cma_alloc_bulk(struct cma *cma, unsigned int align, bool fast,
> +			unsigned int order, size_t nr_requests,
> +			struct page **page_array, size_t *nr_allocated)
> +{
> +	int ret = 0;
> +	size_t i = 0;
> +	unsigned long nr_pages_needed = nr_requests * (1 << order);
> +	unsigned long nr_chunk_pages, nr_pages;
> +	unsigned long mask, offset;
> +	unsigned long pfn = -1;
> +	unsigned long start = 0;
> +	unsigned long bitmap_maxno, bitmap_no, bitmap_count;
> +	struct page *page = NULL;
> +	enum alloc_contig_mode mode = fast ? ALLOC_CONTIG_FAST :
> +						ALLOC_CONTIG_NORMAL;
> +	*nr_allocated = 0;
> +	if (!cma || !cma->count || !cma->bitmap || !page_array)
> +		return -EINVAL;
> +
> +	if (!nr_pages_needed)
> +		return 0;
> +
> +	nr_chunk_pages = 1 << max_t(unsigned int, order, pageblock_order);
> +
> +	mask = cma_bitmap_aligned_mask(cma, align);
> +	offset = cma_bitmap_aligned_offset(cma, align);
> +	bitmap_maxno = cma_bitmap_maxno(cma);
> +
> +	lru_add_drain_all();
> +	drain_all_pages(NULL);
> +
> +	while (nr_pages_needed) {
> +		nr_pages = min(nr_chunk_pages, nr_pages_needed);
> +
> +		bitmap_count = cma_bitmap_pages_to_bits(cma, nr_pages);
> +		mutex_lock(&cma->lock);
> +		bitmap_no = bitmap_find_next_zero_area_off(cma->bitmap,
> +				bitmap_maxno, start, bitmap_count, mask,
> +				offset);
> +		if (bitmap_no >= bitmap_maxno) {
> +			mutex_unlock(&cma->lock);
> +			break;
> +		}
> +		bitmap_set(cma->bitmap, bitmap_no, bitmap_count);
> +		/*
> +		 * It's safe to drop the lock here. If the migration fails
> +		 * cma_clear_bitmap will take the lock again and unmark it.
> +		 */
> +		mutex_unlock(&cma->lock);
> +
> +		pfn = cma->base_pfn + (bitmap_no << cma->order_per_bit);
> +		ret = alloc_contig_range(pfn, pfn + nr_pages, MIGRATE_CMA,
> +						GFP_KERNEL|__GFP_NOWARN, mode);
> +		if (ret) {
> +			cma_clear_bitmap(cma, pfn, nr_pages);
> +			if (ret != -EBUSY)
> +				break;
> +
> +			/* continue to search next block */
> +			start = (pfn + nr_pages - cma->base_pfn) >>
> +						cma->order_per_bit;
> +			continue;
> +		}
> +
> +		page = pfn_to_page(pfn);
> +		while (nr_pages) {
> +			page_array[i++] = page;
> +			reset_page_kasan_tag(page, 1 << order);
> +			page += 1 << order;
> +			nr_pages -= 1 << order;
> +			nr_pages_needed -= 1 << order;
> +		}
> +
> +		start = bitmap_no + bitmap_count;
> +	}
> +
> +	*nr_allocated = i;
> +
> +	if (!ret && nr_pages_needed)
> +		ret = -EBUSY;
> +
> +	return ret;
> +}
> +
>  /**
>   * cma_release() - release allocated pages
>   * @cma:   Contiguous memory region for which the allocation is performed.
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index adfbfd95fbc3..2a1799ff14fc 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8463,7 +8463,8 @@ static unsigned long pfn_max_align_up(unsigned long pfn)
>  
>  /* [start, end) must belong to a single zone. */
>  static int __alloc_contig_migrate_range(struct compact_control *cc,
> -					unsigned long start, unsigned long end)
> +					unsigned long start, unsigned long end,
> +					unsigned int max_tries)
>  {
>  	/* This function is based on compact_zone() from compaction.c. */
>  	unsigned int nr_reclaimed;
> @@ -8491,7 +8492,7 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
>  				break;
>  			}
>  			tries = 0;
> -		} else if (++tries == 5) {
> +		} else if (++tries == max_tries) {
>  			ret = ret < 0 ? ret : -EBUSY;
>  			break;
>  		}
> @@ -8553,6 +8554,7 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  	unsigned long outer_start, outer_end;
>  	unsigned int order;
>  	int ret = 0;
> +	bool fast_mode = mode == ALLOC_CONTIG_FAST;
>  
>  	struct compact_control cc = {
>  		.nr_migratepages = 0,
> @@ -8595,7 +8597,8 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  	if (ret)
>  		return ret;
>  
> -	drain_all_pages(cc.zone);
> +	if (!fast_mode)
> +		drain_all_pages(cc.zone);
>  
>  	/*
>  	 * In case of -EBUSY, we'd like to know which page causes problem.
> @@ -8607,7 +8610,7 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  	 * allocated.  So, if we fall through be sure to clear ret so that
>  	 * -EBUSY is not accidentally used or returned to caller.
>  	 */
> -	ret = __alloc_contig_migrate_range(&cc, start, end);
> +	ret = __alloc_contig_migrate_range(&cc, start, end, fast_mode ? 1 : 5);
>  	if (ret && ret != -EBUSY)
>  		goto done;
>  	ret =0;
> @@ -8629,7 +8632,8 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  	 * isolated thus they won't get removed from buddy.
>  	 */
>  
> -	lru_add_drain_all();
> +	if (!fast_mode)
> +		lru_add_drain_all();
>  
>  	order = 0;
>  	outer_start = start;
> @@ -8656,8 +8660,9 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  
>  	/* Make sure the range is really isolated. */
>  	if (test_pages_isolated(outer_start, end, 0)) {
> -		pr_info_ratelimited("%s: [%lx, %lx) PFNs busy\n",
> -			__func__, outer_start, end);
> +		if (!fast_mode)
> +			pr_info_ratelimited("%s: [%lx, %lx) PFNs busy\n",
> +				__func__, outer_start, end);
>  		ret = -EBUSY;
>  		goto done;
>  	}
> 


-- 
Thanks,

David / dhildenb

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