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Date: Tue, 30 Jan 2024 11:35:47 +0000
From: Alexandru Elisei <alexandru.elisei@arm.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: catalin.marinas@arm.com, will@kernel.org, oliver.upton@linux.dev,
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Subject: Re: [PATCH RFC v3 08/35] mm: cma: Introduce cma_alloc_range()
Message-ID: <ZbjfEzlNgprdxfxX@raptor>
References: <20240125164256.4147-1-alexandru.elisei@arm.com>
 <20240125164256.4147-9-alexandru.elisei@arm.com>
 <61a3dbb7-25b6-4f49-aa70-9a8aaeb53365@arm.com>
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In-Reply-To: <61a3dbb7-25b6-4f49-aa70-9a8aaeb53365@arm.com>

Hi,

On Tue, Jan 30, 2024 at 10:50:00AM +0530, Anshuman Khandual wrote:
> 
> 
> 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 ?

I tried doing that, and I gave up because:

- It made cma_alloc() even more complex and hard to follow.

- What value should 'start_pfn' be to tell cma_alloc() that it should be
  ignored? Or, to put it another way, what pfn number is invalid on **all**
  platforms that Linux supports?

I can give it another go if we can come up with an invalid value for
'start_pfn'.

> 
> 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.

I had the same idea in the previous iteration, David Hildenbrand suggested
this approach [1].

[1] https://lore.kernel.org/linux-fsdevel/2aafd53f-af1f-45f3-a08c-d11962254315@redhat.com/

Thanks,
Alex

> 
> > 
> > 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.
>