Re: [PATCH] mm/page_alloc: Skip non present sections on zone initialization

Kirill A. Shutemov
2020-01-10T14:47:23Z

On Fri, Jan 10, 2020 at 03:34:49PM +0100, David Hildenbrand wrote:
> On 10.01.20 14:45, Kirill A. Shutemov wrote:
> > On Fri, Jan 10, 2020 at 02:15:26PM +0100, David Hildenbrand wrote:
> >> On 08.01.20 15:40, Michal Hocko wrote:
> >>> On Mon 30-12-19 12:38:28, Kirill A. Shutemov wrote:
> >>>> memmap_init_zone() can be called on the ranges with holes during the
> >>>> boot. It will skip any non-valid PFNs one-by-one. It works fine as long
> >>>> as holes are not too big.
> >>>>
> >>>> But huge holes in the memory map causes a problem. It takes over 20
> >>>> seconds to walk 32TiB hole. x86-64 with 5-level paging allows for much
> >>>> larger holes in the memory map which would practically hang the system.
> >>>>
> >>>> Deferred struct page init doesn't help here. It only works on the
> >>>> present ranges.
> >>>>
> >>>> Skipping non-present sections would fix the issue.
> >>>
> >>> Makes sense to me.
> >>>
> >>>> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> >>>
> >>> That pfn inc back and forth is quite ugly TBH but whatever.
> >>
> >> Indeed, can we please rewrite the loop to fix that?
> >
> > Any suggestions?
> >
> > I don't see an obvious way to not break readablity in another place.
> >
>
> I'd probably do it like this (applied some other tweaks, untested)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index cb766aac6772..a96b1ad1d74b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5859,6 +5859,22 @@ overlap_memmap_init(unsigned long zone, unsigned long *pfn)
> return false;
> }
>
> +static inline __meminit unsigned long next_present_pfn(unsigned long pfn)
> +{
> +#ifdef CONFIG_SPARSEMEM

I would rather keep it around function, but it's matter of taste.

> + unsigned long section_nr = pfn_to_section_nr(pfn + 1);
> +
> + /*
> + * Note: We don't check the subsection bitmap, so this can produce
> + * false positives when only subsections are present/valid. The
> + * caller should recheck if the returned pfn is valid.
> + */
> + if (!present_section_nr(section_nr))
> + return section_nr_to_pfn(next_present_section_nr(section_nr));

This won't compile. next_present_section_nr() is static to mm/sparse.c.

> +#endif
> + return pfn++;
> +}
> +
> /*
> * Initially all pages are reserved - free ones are freed
> * up by memblock_free_all() once the early boot process is
> @@ -5892,18 +5908,22 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
> }
> #endif
>
> - for (pfn = start_pfn; pfn < end_pfn; pfn++) {
> + pfn = start_pfn;
> + while (pfn < end_pfn) {
> /*
> * There can be holes in boot-time mem_map[]s handed to this
> * function. They do not exist on hotplugged memory.
> */
> if (context == MEMMAP_EARLY) {
> - if (!early_pfn_valid(pfn))
> + if (!early_pfn_valid(pfn)) {
> + pfn = next_present_pfn(pfn, end_pfn);
> continue;
> - if (!early_pfn_in_nid(pfn, nid))
> - continue;
> - if (overlap_memmap_init(zone, &pfn))
> + }
> + if (!early_pfn_in_nid(pfn, nid) ||
> + overlap_memmap_init(zone, &pfn)) {
> + pfn++;
> continue;
> + }
> if (defer_init(nid, pfn, end_pfn))
> break;
> }
> @@ -5929,6 +5949,7 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
> set_pageblock_migratetype(page, MIGRATE_MOVABLE);
> cond_resched();
> }
> + pfn++;
> }
>
>
> I played with using a "pfn = next_init_pfn()" in the for loop instead, moving all
> the checks in there, but didn't turn out too well.

Well, it's better than I thought, but... I'm fine either way.

--
Kirill A. Shutemov