Re: [PATCH RFC v3 08/35] mm: cma: Introduce cma_alloc_range()

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 1/25/24 22:12, Alexandru Elisei wrote:
> Today, cma_alloc() is used to allocate a contiguous memory region. The
> function allows the caller to specify the number of pages to allocate, but
> not the starting address. cma_alloc() will walk over the entire CMA region
> trying to allocate the first available range of the specified size.
> 
> Introduce cma_alloc_range(), which makes CMA more versatile by allowing the
> caller to specify a particular range in the CMA region, defined by the
> start pfn and the size.
> 
> arm64 will make use of this function when tag storage management will be
> implemented: cma_alloc_range() will be used to reserve the tag storage
> associated with a tagged page.

Basically, you would like to pass on a preferred start address and the
allocation could just fail if a contig range is not available from such
a starting address ?

Then why not just change cma_alloc() to take a new argument 'start_pfn'.
Why create a new but almost similar allocator ?

But then I am wondering why this could not be done in the arm64 platform
code itself operating on a CMA area reserved just for tag storage. Unless
this new allocator has other usage beyond MTE, this could be implemented
in the platform itself.

> 
> Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> ---
> 
> Changes since rfc v2:
> 
> * New patch.
> 
>  include/linux/cma.h        |  2 +
>  include/trace/events/cma.h | 59 ++++++++++++++++++++++++++
>  mm/cma.c                   | 86 ++++++++++++++++++++++++++++++++++++++
>  3 files changed, 147 insertions(+)
> 
> diff --git a/include/linux/cma.h b/include/linux/cma.h
> index 63873b93deaa..e32559da6942 100644
> --- a/include/linux/cma.h
> +++ b/include/linux/cma.h
> @@ -50,6 +50,8 @@ 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, unsigned long count, unsigned int align,
>  			      bool no_warn);
> +extern int cma_alloc_range(struct cma *cma, unsigned long start, unsigned long count,
> +			   unsigned tries, gfp_t gfp);
>  extern bool cma_pages_valid(struct cma *cma, const struct page *pages, unsigned long count);
>  extern bool cma_release(struct cma *cma, const struct page *pages, unsigned long count);
>  
> diff --git a/include/trace/events/cma.h b/include/trace/events/cma.h
> index 25103e67737c..a89af313a572 100644
> --- a/include/trace/events/cma.h
> +++ b/include/trace/events/cma.h
> @@ -36,6 +36,65 @@ TRACE_EVENT(cma_release,
>  		  __entry->count)
>  );
>  
> +TRACE_EVENT(cma_alloc_range_start,
> +
> +	TP_PROTO(const char *name, unsigned long start, unsigned long count,
> +		 unsigned tries),
> +
> +	TP_ARGS(name, start, count, tries),
> +
> +	TP_STRUCT__entry(
> +		__string(name, name)
> +		__field(unsigned long, start)
> +		__field(unsigned long, count)
> +		__field(unsigned, tries)
> +	),
> +
> +	TP_fast_assign(
> +		__assign_str(name, name);
> +		__entry->start = start;
> +		__entry->count = count;
> +		__entry->tries = tries;
> +	),
> +
> +	TP_printk("name=%s start=%lx count=%lu tries=%u",
> +		  __get_str(name),
> +		  __entry->start,
> +		  __entry->count,
> +		  __entry->tries)
> +);
> +
> +TRACE_EVENT(cma_alloc_range_finish,
> +
> +	TP_PROTO(const char *name, unsigned long start, unsigned long count,
> +		 unsigned attempts, int err),
> +
> +	TP_ARGS(name, start, count, attempts, err),
> +
> +	TP_STRUCT__entry(
> +		__string(name, name)
> +		__field(unsigned long, start)
> +		__field(unsigned long, count)
> +		__field(unsigned, attempts)
> +		__field(int, err)
> +	),
> +
> +	TP_fast_assign(
> +		__assign_str(name, name);
> +		__entry->start = start;
> +		__entry->count = count;
> +		__entry->attempts = attempts;
> +		__entry->err = err;
> +	),
> +
> +	TP_printk("name=%s start=%lx count=%lu attempts=%u err=%d",
> +		  __get_str(name),
> +		  __entry->start,
> +		  __entry->count,
> +		  __entry->attempts,
> +		  __entry->err)
> +);
> +
>  TRACE_EVENT(cma_alloc_start,
>  
>  	TP_PROTO(const char *name, unsigned long count, unsigned int align),
> diff --git a/mm/cma.c b/mm/cma.c
> index 543bb6b3be8e..4a0f68b9443b 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -416,6 +416,92 @@ static void cma_debug_show_areas(struct cma *cma)
>  static inline void cma_debug_show_areas(struct cma *cma) { }
>  #endif
>  
> +/**
> + * cma_alloc_range() - allocate pages in a specific range
> + * @cma:   Contiguous memory region for which the allocation is performed.
> + * @start: Starting pfn of the allocation.
> + * @count: Requested number of pages
> + * @tries: Number of tries if the range is busy
> + * @no_warn: Avoid printing message about failed allocation
> + *
> + * This function allocates part of contiguous memory from a specific contiguous
> + * memory area, from the specified starting address. The 'start' pfn and the the
> + * 'count' number of pages must be aligned to the CMA bitmap order per bit.
> + */
> +int cma_alloc_range(struct cma *cma, unsigned long start, unsigned long count,
> +		    unsigned tries, gfp_t gfp)
> +{
> +	unsigned long bitmap_maxno, bitmap_no, bitmap_start, bitmap_count;
> +	unsigned long i = 0;
> +	struct page *page;
> +	int err = -EINVAL;
> +
> +	if (!cma || !cma->count || !cma->bitmap)
> +		goto out_stats;
> +
> +	trace_cma_alloc_range_start(cma->name, start, count, tries);
> +
> +	if (!count || start < cma->base_pfn ||
> +	    start + count > cma->base_pfn + cma->count)
> +		goto out_stats;
> +
> +	if (!IS_ALIGNED(start | count, 1 << cma->order_per_bit))
> +		goto out_stats;
> +
> +	bitmap_start = (start - cma->base_pfn) >> cma->order_per_bit;
> +	bitmap_maxno = cma_bitmap_maxno(cma);
> +	bitmap_count = cma_bitmap_pages_to_bits(cma, count);
> +
> +	spin_lock_irq(&cma->lock);
> +	bitmap_no = bitmap_find_next_zero_area(cma->bitmap, bitmap_maxno,
> +					       bitmap_start, bitmap_count, 0);
> +	if (bitmap_no != bitmap_start) {
> +		spin_unlock_irq(&cma->lock);
> +		err = -EEXIST;
> +		goto out_stats;
> +	}
> +	bitmap_set(cma->bitmap, bitmap_start, bitmap_count);
> +	spin_unlock_irq(&cma->lock);
> +
> +	for (i = 0; i < tries; i++) {
> +		mutex_lock(&cma_mutex);
> +		err = alloc_contig_range(start, start + count, MIGRATE_CMA, gfp);
> +		mutex_unlock(&cma_mutex);
> +
> +		if (err != -EBUSY)
> +			break;
> +	}
> +
> +	if (err) {
> +		cma_clear_bitmap(cma, start, count);
> +	} else {
> +		page = pfn_to_page(start);
> +
> +		/*
> +		 * CMA can allocate multiple page blocks, which results in
> +		 * different blocks being marked with different tags. Reset the
> +		 * tags to ignore those page blocks.
> +		 */
> +		for (i = 0; i < count; i++)
> +			page_kasan_tag_reset(nth_page(page, i));
> +	}
> +
> +out_stats:
> +	trace_cma_alloc_range_finish(cma->name, start, count, i, err);
> +
> +	if (err) {
> +		count_vm_events(CMA_ALLOC_FAIL, count);
> +		if (cma)
> +			cma_sysfs_account_fail_pages(cma, count);
> +	} else {
> +		count_vm_events(CMA_ALLOC_SUCCESS, count);
> +		cma_sysfs_account_success_pages(cma, count);
> +	}
> +
> +	return err;
> +}
> +
> +
>  /**
>   * cma_alloc() - allocate pages from contiguous area
>   * @cma:   Contiguous memory region for which the allocation is performed.