Re: [External] Re: [PATCH v15 4/8] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

[Muchun Song <songmuchun@bytedance.com>]

On Fri, Feb 12, 2021 at 11:32 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Mon 08-02-21 16:50:09, Muchun Song wrote:
> > When we free a HugeTLB page to the buddy allocator, we should allocate the
> > vmemmap pages associated with it. But we may cannot allocate vmemmap pages
> > when the system is under memory pressure, in this case, we just refuse to
> > free the HugeTLB page instead of looping forever trying to allocate the
> > pages.
>
> Thanks for simplifying the implementation from your early proposal!
>
> This will not be looping for ever. The allocation will usually trigger
> the OOM killer and sooner or later there will be a memory to allocate
> from or the system panics when there are no eligible tasks to kill. This
> is just a side note.
>
> I think the changelog could benefit from a more explicit documentation
> of those error failures. There are different cases when the hugetlb page
> is freed. It can be due to an admin intervention (decrease the pool),
> overcommit, migration, dissolving and likely some others. Most of them
> should be fine to stay in the pool which would just increase the surplus
> pages in the pool. I am not so sure about dissolving path.

Thanks. I will update the changelog.

> [...]
> > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > index 0209b736e0b4..3d85e3ab7caa 100644
> > --- a/mm/hugetlb_vmemmap.c
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -169,6 +169,8 @@
> >   * (last) level. So this type of HugeTLB page can be optimized only when its
> >   * size of the struct page structs is greater than 2 pages.
> >   */
> > +#define pr_fmt(fmt)  "HugeTLB: " fmt
> > +
> >  #include "hugetlb_vmemmap.h"
> >
> >  /*
> > @@ -198,6 +200,34 @@ static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
> >       return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
> >  }
> >
> > +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> > +{
> > +     int ret;
> > +     unsigned long vmemmap_addr = (unsigned long)head;
> > +     unsigned long vmemmap_end, vmemmap_reuse;
> > +
> > +     if (!free_vmemmap_pages_per_hpage(h))
> > +             return 0;
> > +
> > +     vmemmap_addr += RESERVE_VMEMMAP_SIZE;
> > +     vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
> > +     vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
> > +
> > +     /*
> > +      * The pages which the vmemmap virtual address range [@vmemmap_addr,
> > +      * @vmemmap_end) are mapped to are freed to the buddy allocator, and
> > +      * the range is mapped to the page which @vmemmap_reuse is mapped to.
> > +      * When a HugeTLB page is freed to the buddy allocator, previously
> > +      * discarded vmemmap pages must be allocated and remapping.
> > +      */
> > +     ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
> > +                               GFP_ATOMIC | __GFP_NOWARN | __GFP_THISNODE);
>
> I do not think that this is a good allocation mode. GFP_ATOMIC is a non
> sleeping allocation and a medium memory pressure might cause it to
> fail prematurely. I do not think this is really an atomic context which
> couldn't afford memory reclaim. I also do not think we want to grant

Because alloc_huge_page_vmemmap is called under hugetlb_lock
now. So using GFP_ATOMIC indeed makes the code more simpler.
From the document of the kernel, I learned that __GFP_NOMEMALLOC
can be used to explicitly forbid access to emergency reserves. So if
we do not want to use the reserve memory. How about replacing it to

GFP_ATOMIC | __GFP_NOMEMALLOC | __GFP_NOWARN | __GFP_THISNODE

Thanks.

> access to memory reserve is reasonable. Just think of a huge number of
> hugetlb pages being freed which can deplete the memory reserve for
> atomic allocations. I think that you want
>         GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN | __GFP_THISNODE
>
> for an initial implementation. The justification being that the
> allocation should at least try to reclaim but it shouldn't cause any
> major disruption because the failure is not fatal. If the failure rate
> would be impractically high then just drop NORETRY part. You can replace
> it by __GFP_RETRY_MAYFAIL but that shouldn't be strictly necessary
> because __GFP_THISNODE on its own implies on OOM killer, but that is
> kinda ugly to rely on.
> --
> Michal Hocko
> SUSE Labs