Re: [PATCH] mm: hugetlb: avoid fallback for specific node allocation of 1G pages

[Luiz Capitulino <luizcap@redhat.com>]

On 2025-02-11 04:06, Oscar Salvador wrote:
> On Mon, Feb 10, 2025 at 10:48:56PM -0500, Luiz Capitulino wrote:
>> When using the HugeTLB kernel command-line to allocate 1G pages from
>> a specific node, such as:
>>
>>     default_hugepagesz=1G hugepages=1:1
>>
>> If node 1 happens to not have enough memory for the requested number of
>> 1G pages, the allocation falls back to other nodes. A quick way to
>> reproduce this is by creating a KVM guest with a memory-less node and
>> trying to allocate 1 1G page from it. Instead of failing, the allocation
>> will fallback to other nodes.
>>
>> This defeats the purpose of node specific allocation. Also, specific
>> node allocation for 2M pages don't have this behavior: the allocation
>> will just fail for the pages it can't satisfy.
>>
>> This issue happens because HugeTLB calls memblock_alloc_try_nid_raw()
>> for 1G boot-time allocation as this function falls back to other nodes
>> if the allocation can't be satisfied. Use memblock_alloc_exact_nid_raw()
>> instead, which ensures that the allocation will only be satisfied from
>> the specified node.
>>
>> Fixes: b5389086ad7b ("hugetlbfs: extend the definition of hugepages parameter to support node allocation")
>>
>> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
> 
> Acked-by: Oscar Salvador <osalvador@suse.de>
> 
> This was discussed yesterday in [1], ccing Frank for awareness.
> 
> [1] https://patchwork.kernel.org/project/linux-mm/patch/20250206185109.1210657-6-fvdl@google.com/

Interesting, thanks for the reference.

I stumbled over this issue back in December when debugging a HugeTLB issue
at Red Hat (David knows it ;) ) and had this patch pending for more than a
week now...