Re: [PATCH mm-unstable 6/8] mm/hugetlb: convert alloc_migrate_huge_page to folios

Sidhartha Kumar
2023-01-09T16:28:01Z

On 1/6/23 6:54 PM, Mike Kravetz wrote:
> On 01/03/23 13:13, Sidhartha Kumar wrote:
>> Change alloc_huge_page_nodemask() to alloc_hugetlb_folio_nodemask() and
>> alloc_migrate_huge_page() to alloc_migrate_hugetlb_folio(). Both functions
>> now return a folio rather than a page.
>
>> /* mempolicy aware migration callback */
>> @@ -2357,16 +2357,16 @@ struct page *alloc_huge_page_vma(struct hstate *h, struct vm_area_struct *vma,
>> {
>> struct mempolicy *mpol;
>> nodemask_t *nodemask;
>> - struct page *page;
>> + struct folio *folio;
>> gfp_t gfp_mask;
>> int node;
>>
>> gfp_mask = htlb_alloc_mask(h);
>> node = huge_node(vma, address, gfp_mask, &mpol, &nodemask);
>> - page = alloc_huge_page_nodemask(h, node, nodemask, gfp_mask);
>> + folio = alloc_hugetlb_folio_nodemask(h, node, nodemask, gfp_mask);
>> mpol_cond_put(mpol);
>>
>> - return page;
>> + return &folio->page;
>
> Is it possible that folio could be NULL here and cause addressing exception?
>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 6932b3d5a9dd..fab706b78be1 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1622,6 +1622,7 @@ struct page *alloc_migration_target(struct page *page, unsigned long private)
>> struct migration_target_control *mtc;
>> gfp_t gfp_mask;
>> unsigned int order = 0;
>> + struct folio *hugetlb_folio = NULL;
>> struct folio *new_folio = NULL;
>> int nid;
>> int zidx;
>> @@ -1636,7 +1637,9 @@ struct page *alloc_migration_target(struct page *page, unsigned long private)
>> struct hstate *h = folio_hstate(folio);
>>
>> gfp_mask = htlb_modify_alloc_mask(h, gfp_mask);
>> - return alloc_huge_page_nodemask(h, nid, mtc->nmask, gfp_mask);
>> + hugetlb_folio = alloc_hugetlb_folio_nodemask(h, nid,
>> + mtc->nmask, gfp_mask);
>> + return &hugetlb_folio->page;
>
> and, here as well?

Hi Mike,

It is possible that the folio could be null but I believe these
instances would not cause an addressing exception because as described
in [1], &folio->page is safe even if the folio is NULL as the page
offset is at 0.

[1] https://lore.kernel.org/lkml/Y7h4jsv6jl0XSIsk@casper.infradead.org/T/