Messages in this thread | | | Date | Fri, 2 Feb 2024 09:35:58 +0800 | Subject | Re: [RFC PATCH] mm: hugetlb: remove __GFP_THISNODE flag when dissolving the old hugetlb | From | Baolin Wang <> |
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On 2/1/2024 11:27 PM, Michal Hocko wrote: > On Thu 01-02-24 21:31:13, Baolin Wang wrote: >> Since commit 369fa227c219 ("mm: make alloc_contig_range handle free >> hugetlb pages"), the alloc_contig_range() can handle free hugetlb pages >> by allocating a new fresh hugepage, and replacing the old one in the >> free hugepage pool. >> >> However, our customers can still see the failure of alloc_contig_range() >> when seeing a free hugetlb page. The reason is that, there are few memory >> on the old hugetlb page's node, and it can not allocate a fresh hugetlb >> page on the old hugetlb page's node in isolate_or_dissolve_huge_page() with >> setting __GFP_THISNODE flag. This makes sense to some degree. >> >> Later, the commit ae37c7ff79f1 (" mm: make alloc_contig_range handle >> in-use hugetlb pages") handles the in-use hugetlb pages by isolating it >> and doing migration in __alloc_contig_migrate_range(), but it can allow >> fallbacking to other numa node when allocating a new hugetlb in >> alloc_migration_target(). >> >> This introduces inconsistency to handling free and in-use hugetlb. >> Considering the CMA allocation and memory hotplug relying on the >> alloc_contig_range() are important in some scenarios, as well as keeping >> the consistent hugetlb handling, we should remove the __GFP_THISNODE flag >> in isolate_or_dissolve_huge_page() to allow fallbacking to other numa node, >> which can solve the failure of alloc_contig_range() in our case. > > I do agree that the inconsistency is not really good but I am not sure > dropping __GFP_THISNODE is the right way forward. Breaking pre-allocated > per-node pools might result in unexpected failures when node bound > workloads doesn't get what is asssumed available. Keep in mind that our > user APIs allow to pre-allocate per-node pools separately.
Yes, I agree, that is also what I concered. But sometimes users don't care about the distribution of per-node hugetlb, instead they are more concerned about the success of cma allocation or memory hotplug.
> The in-use hugetlb is a very similar case. While having a temporarily > misplaced page doesn't really look terrible once that hugetlb page is > released back into the pool we are back to the case above. Either we > make sure that the node affinity is restored later on or it shouldn't be > migrated to a different node at all.
Agree. So how about below changing? (1) disallow fallbacking to other nodes when handing in-use hugetlb, which can ensure consistent behavior in handling hugetlb. (2) introduce a new sysctl (may be named as "hugetlb_allow_fallback_nodes") for users to control to allow fallbacking, that can solve the CMA or memory hotplug failures that users are more concerned about.
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