Re: [External] Re: [PATCH v19 7/8] mm: hugetlb: add a kernel parameter hugetlb_free_vmemmap

[David Hildenbrand <david@redhat.com>]

On 19.03.21 13:15, Muchun Song wrote:
> On Fri, Mar 19, 2021 at 4:59 PM Oscar Salvador <osalvador@suse.de> wrote:
>>
>> On Mon, Mar 15, 2021 at 05:20:14PM +0800, Muchun Song wrote:
>>> --- a/arch/x86/mm/init_64.c
>>> +++ b/arch/x86/mm/init_64.c
>>> @@ -34,6 +34,7 @@
>>>   #include <linux/gfp.h>
>>>   #include <linux/kcore.h>
>>>   #include <linux/bootmem_info.h>
>>> +#include <linux/hugetlb.h>
>>>
>>>   #include <asm/processor.h>
>>>   #include <asm/bios_ebda.h>
>>> @@ -1557,7 +1558,8 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>>>   {
>>>        int err;
>>>
>>> -     if (end - start < PAGES_PER_SECTION * sizeof(struct page))
>>> +     if ((is_hugetlb_free_vmemmap_enabled()  && !altmap) ||
>>> +         end - start < PAGES_PER_SECTION * sizeof(struct page))
>>>                err = vmemmap_populate_basepages(start, end, node, NULL);
>>>        else if (boot_cpu_has(X86_FEATURE_PSE))
>>>                err = vmemmap_populate_hugepages(start, end, node, altmap);
>>
>> I've been thinking about this some more.
>>
>> Assume you opt-in the hugetlb-vmemmap feature, and assume you pass a valid altmap
>> to vmemmap_populate.
>> This will lead to use populating the vmemmap array with hugepages.
> 
> Right.
> 
>>
>> What if then, a HugeTLB gets allocated and falls within that memory range (backed
>> by hugetpages)?
> 
> I am not sure whether we can allocate the HugeTLB pages from there.
> Will only device memory pass a valid altmap parameter to
> vmemmap_populate()? If yes, can we allocate HugeTLB pages from
> device memory? Sorry, I am not an expert on this.

I think, right now, yes. System RAM that's applicable for HugePages 
never uses an altmap. But Oscar's patch will change that, maybe before 
your series might get included from what I've been reading. [1]

[1] https://lkml.kernel.org/r/20210319092635.6214-1-osalvador@suse.de

> 
> 
>> AFAIK, this will get us in trouble as currently the code can only operate on memory
>> backed by PAGE_SIZE pages, right?
>>
>> I cannot remember, but I do not think nothing prevents that from happening?
>> Am I missing anything?
> 
> Maybe David H is more familiar with this.
> 
> Hi David,
> 
> Do you have some suggestions on this?

There has to be some way to identify whether we can optimize specific 
vmemmap pages or should just leave them alone. altmap vs. !altmap.

Unfortunately, there is no easy way to detect that - e.g., 
PageReserved() applies also to boot memory.

We could go back to setting a special PageType for these vmemmap pages, 
indicating "this is a page allocated from an altmap, don't touch it".

-- 
Thanks,

David / dhildenb