linux-kernel - Re: [RFC PATCH v2 22/51] mm: hugetlb: Refactor hugetlb allocation functions

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Message-ID: <diqzv7ozmh82.fsf@ackerleytng-ctop.c.googlers.com>
Date: Fri, 13 Jun 2025 15:03:57 -0700
From: Ackerley Tng <ackerleytng@...gle.com>
To: Ira Weiny <ira.weiny@...el.com>
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Subject: Re: [RFC PATCH v2 22/51] mm: hugetlb: Refactor hugetlb allocation functions

Ira Weiny <ira.weiny@...el.com> writes:

> Ackerley Tng wrote:
>> Refactor dequeue_hugetlb_folio() and alloc_surplus_hugetlb_folio() to
>> take mpol, nid and nodemask. This decouples allocation of a folio from
>> a vma.
>> 
>> Signed-off-by: Ackerley Tng <ackerleytng@...gle.com>
>> Change-Id: I890fb46fe8c6349383d8cf89befc68a4994eb416
>> ---
>>  mm/hugetlb.c | 64 ++++++++++++++++++++++++----------------------------
>>  1 file changed, 30 insertions(+), 34 deletions(-)
>> 
>
> [snip]
>
>>  
>> @@ -2993,6 +2974,11 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
>>  	int ret, idx;
>>  	struct hugetlb_cgroup *h_cg = NULL;
>>  	gfp_t gfp = htlb_alloc_mask(h) | __GFP_RETRY_MAYFAIL;
>> +	struct mempolicy *mpol;
>> +	nodemask_t *nodemask;
>> +	gfp_t gfp_mask;
>> +	pgoff_t ilx;
>> +	int nid;
>>  
>>  	idx = hstate_index(h);
>>  
>> @@ -3032,7 +3018,6 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
>>  
>>  		subpool_reservation_exists = npages_req == 0;
>>  	}
>> -
>>  	reservation_exists = vma_reservation_exists || subpool_reservation_exists;
>>  
>>  	/*
>> @@ -3048,21 +3033,30 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
>>  			goto out_subpool_put;
>>  	}
>>  
>> +	mpol = get_vma_policy(vma, addr, h->order, &ilx);
>
> Why does the memory policy need to be acquired here instead of after the
> cgroup charge?  AFAICT this is not needed and would at least eliminate 1
> of the error conditions puts.
>

I was hoping that by taking this early, the upcoming refactoring out of
hugetlb_alloc_folio() will look like a nice, clean removal of the middle
of this function, leaving acquiring of the mpol and mpol_cond_put()
in-place.

In the next revision I'm splitting up the refactoring in this patch
further so if this is still an issue in some number of revisions' time,
I can fix this.

>> +
>>  	ret = hugetlb_cgroup_charge_cgroup(idx, pages_per_huge_page(h), &h_cg);
>> -	if (ret)
>> +	if (ret) {
>> +		mpol_cond_put(mpol);
>                 ^^^^
> 		here
>
> All that said I think the use of some new cleanup macros could really help
> a lot of this code.
>

I'm happy to try that out...

> What do folks in this area of the kernel think of those?
>

not sure though.

> Ira
>
> [snip]