Date: Wed, 31 Jan 2024 11:54:17 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Alexandru Elisei <alexandru.elisei@....com>
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Subject: Re: [PATCH RFC v3 08/35] mm: cma: Introduce cma_alloc_range()



On 1/30/24 17:05, Alexandru Elisei wrote:
> 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'.

Something negative might work. How about -1/-1UL ? A quick search gives
some instances such as ...

git grep "pfn == -1"

mm/mm_init.c:   if (*start_pfn == -1UL)
mm/vmscan.c:            if (pfn == -1)
mm/vmscan.c:            if (pfn == -1)
mm/vmscan.c:            if (pfn == -1)
tools/testing/selftests/mm/hugepage-vmemmap.c:  if (pfn == -1UL) {

Could not -1UL be abstracted as common macro MM_INVALID_PFN to be used in
such scenarios including here ?

> 
>> 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/

There are two different cma_alloc() proposals here - including the next
patch i.e mm: cma: Fast track allocating memory when the pages are free

1) Augment cma_alloc() or add cma_alloc_range() with start_pfn parameter
2) Speed up cma_alloc() for small allocation requests when pages are free

The second one if separated out from this series could be considered on
its own as it will help all existing cma_alloc() callers. The first one
definitely needs an use case as provided in this series.

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