Re: [PATCH] mm: page_alloc: Skip regions with hugetlbfs pages when allocating 1G pages

[Oscar Salvador]

On 2023-04-14 16:14, Mel Gorman wrote:
> A bug was reported by Yuanxi Liu where allocating 1G pages at runtime 
> is
> taking an excessive amount of time for large amounts of memory. Further
> testing allocating huge pages that the cost is linear i.e. if 
> allocating
> 1G pages in batches of 10 then the time to allocate nr_hugepages from
> 10->20->30->etc increases linearly even though 10 pages are allocated 
> at
> each step. Profiles indicated that much of the time is spent checking 
> the
> validity within already existing huge pages and then attempting a 
> migration
> that fails after isolating the range, draining pages and a whole lot of
> other useless work.
>
> Commit eb14d4eefdc4 ("mm,page_alloc: drop unnecessary checks from
> pfn_range_valid_contig") removed two checks, one which ignored huge 
> pages
> for contiguous allocations as huge pages can sometimes migrate. While
> there may be value on migrating a 2M page to satisfy a 1G allocation, 
> it's
> potentially expensive if the 1G allocation fails and it's pointless to
> try moving a 1G page for a new 1G allocation or scan the tail pages for
> valid PFNs.
>
> Reintroduce the PageHuge check and assume any contiguous region with
> hugetlbfs pages is unsuitable for a new 1G allocation.
>
> The hpagealloc test allocates huge pages in batches and reports the
> average latency per page over time. This test happens just after boot 
> when
> fragmentation is not an issue. Units are in milliseconds.
...
> BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=217022
> Fixes: eb14d4eefdc4 ("mm,page_alloc: drop unnecessary checks from
> pfn_range_valid_contig")
> Reported-by: Yuanxi Liu <y.liu@xxxxxxxxxxx>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>

Thanks Mel!

--
Oscar Salvador
SUSE Labs