Re: [PATCH v3 1/2] mm: Make alloc_contig_range handle free hugetlb pages

[David Hildenbrand <david@redhat.com>]

On 22.02.21 14:51, Oscar Salvador wrote:
> alloc_contig_range will fail if it ever sees a HugeTLB page within the
> range we are trying to allocate, even when that page is free and can be
> easily reallocated.
> This has proved to be problematic for some users of alloc_contic_range,
> e.g: CMA and virtio-mem, where those would fail the call even when those
> pages lay in ZONE_MOVABLE and are free.
> 
> We can do better by trying to replace such page.
> 
> Free hugepages are tricky to handle so as to no userspace application
> notices disruption, we need to replace the current free hugepage with
> a new one.
> 
> In order to do that, a new function called alloc_and_dissolve_huge_page
> is introduced.
> This function will first try to get a new fresh hugepage, and if it
> succeeds, it will replace the old one in the free hugepage pool.
> 
> All operations are being handled under hugetlb_lock, so no races are
> possible. The only exception is when page's refcount is 0, but it still
> has not been flagged as PageHugeFreed.
> In this case we retry as the window race is quite small and we have high
> chances to succeed next time.
> 
> With regard to the allocation, we restrict it to the node the page belongs
> to with __GFP_THISNODE, meaning we do not fallback on other node's zones.
> 
> Note that gigantic hugetlb pages are fenced off since there is a cyclic
> dependency between them and alloc_contig_range.
> 
> Signed-off-by: Oscar Salvador <osalvador@suse.de>
> ---
>   include/linux/hugetlb.h |   6 +++
>   mm/compaction.c         |  12 ++++++
>   mm/hugetlb.c            | 111 +++++++++++++++++++++++++++++++++++++++++++++++-
>   3 files changed, 127 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index b5807f23caf8..72352d718829 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -505,6 +505,7 @@ struct huge_bootmem_page {
>   	struct hstate *hstate;
>   };
>   
> +bool isolate_or_dissolve_huge_page(struct page *page);
>   struct page *alloc_huge_page(struct vm_area_struct *vma,
>   				unsigned long addr, int avoid_reserve);
>   struct page *alloc_huge_page_nodemask(struct hstate *h, int preferred_nid,
> @@ -775,6 +776,11 @@ void set_page_huge_active(struct page *page);
>   #else	/* CONFIG_HUGETLB_PAGE */
>   struct hstate {};
>   
> +static inline bool isolate_or_dissolve_huge_page(struct page *page)
> +{
> +	return false;
> +}
> +
>   static inline struct page *alloc_huge_page(struct vm_area_struct *vma,
>   					   unsigned long addr,
>   					   int avoid_reserve)
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 190ccdaa6c19..d52506ed9db7 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -905,6 +905,18 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>   			valid_page = page;
>   		}
>   
> +		if (PageHuge(page) && cc->alloc_contig) {
> +			if (!isolate_or_dissolve_huge_page(page))
> +				goto isolate_fail;


So, the callchain is:

alloc_contig_range()->__alloc_contig_migrate_range()->isolate_migratepages_range()->isolate_migratepages_block()

The case I am thinking about is if we run out of memory and would return 
-ENOMEM from alloc_and_dissolve_huge_page(). We silently drop the real 
error (e.g., -ENOMEM vs. -EBUSY vs. e.g., -EAGAIN) we had in 
isolate_or_dissolve_huge_page().


I think we should not swallo such return values in 
isolate_or_dissolve_huge_page() and instead properly report esp. -ENOMEM 
properly up this callchain now. Otherwise we'll end up retrying / 
reporting -EBUSY, which is misleading.

 From isolate_migratepages_range()/isolate_migratepages_block() we'll 
keep reporting "pfn > 0".

a) In isolate_migratepages_range() we'll keep iterating over pageblocks 
although we should just fail with -ENOMEM right away.

b) In __alloc_contig_migrate_range() we'll keep retrying up to 5 times 
although we should just fail with -ENOMEM. We end up returning "-EBUSY" 
after retrying.

c) In alloc_contig_range() we'll continue trying to isolate although we 
should just return -ENOMEM.


I think we have should start returning proper errors from 
isolate_migratepages_range()/isolate_migratepages_block() on critical 
issues (-EINTR, -ENOMEM) instead of going via "!pfn vs. pfn" and 
retrying on "pfn".

So we should then fail with -ENOMEM during isolate_migratepages_range() 
cleanly, just as we would do when we get -ENOMEM during migrate_pages().



-- 
Thanks,

David / dhildenb