Re: [External] Re: [PATCH v13 05/12] mm: hugetlb: allocate the vmemmap pages associated with each HugeTLB page

[Muchun Song <songmuchun@bytedance.com>]

On Mon, Jan 25, 2021 at 5:15 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 25.01.21 08:41, Muchun Song wrote:
> > On Mon, Jan 25, 2021 at 2:40 PM Muchun Song <songmuchun@bytedance.com> wrote:
> >>
> >> On Mon, Jan 25, 2021 at 8:05 AM David Rientjes <rientjes@google.com> wrote:
> >>>
> >>>
> >>> On Sun, 17 Jan 2021, Muchun Song wrote:
> >>>
> >>>> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> >>>> index ce4be1fa93c2..3b146d5949f3 100644
> >>>> --- a/mm/sparse-vmemmap.c
> >>>> +++ b/mm/sparse-vmemmap.c
> >>>> @@ -29,6 +29,7 @@
> >>>>  #include <linux/sched.h>
> >>>>  #include <linux/pgtable.h>
> >>>>  #include <linux/bootmem_info.h>
> >>>> +#include <linux/delay.h>
> >>>>
> >>>>  #include <asm/dma.h>
> >>>>  #include <asm/pgalloc.h>
> >>>> @@ -40,7 +41,8 @@
> >>>>   * @remap_pte:               called for each non-empty PTE (lowest-level) entry.
> >>>>   * @reuse_page:              the page which is reused for the tail vmemmap pages.
> >>>>   * @reuse_addr:              the virtual address of the @reuse_page page.
> >>>> - * @vmemmap_pages:   the list head of the vmemmap pages that can be freed.
> >>>> + * @vmemmap_pages:   the list head of the vmemmap pages that can be freed
> >>>> + *                   or is mapped from.
> >>>>   */
> >>>>  struct vmemmap_remap_walk {
> >>>>       void (*remap_pte)(pte_t *pte, unsigned long addr,
> >>>> @@ -50,6 +52,10 @@ struct vmemmap_remap_walk {
> >>>>       struct list_head *vmemmap_pages;
> >>>>  };
> >>>>
> >>>> +/* The gfp mask of allocating vmemmap page */
> >>>> +#define GFP_VMEMMAP_PAGE             \
> >>>> +     (GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOWARN | __GFP_THISNODE)
> >>>> +
> >>>
> >>> This is unnecessary, just use the gfp mask directly in allocator.
> >>
> >> Will do. Thanks.
> >>
> >>>
> >>>>  static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
> >>>>                             unsigned long end,
> >>>>                             struct vmemmap_remap_walk *walk)
> >>>> @@ -228,6 +234,75 @@ void vmemmap_remap_free(unsigned long start, unsigned long end,
> >>>>       free_vmemmap_page_list(&vmemmap_pages);
> >>>>  }
> >>>>
> >>>> +static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
> >>>> +                             struct vmemmap_remap_walk *walk)
> >>>> +{
> >>>> +     pgprot_t pgprot = PAGE_KERNEL;
> >>>> +     struct page *page;
> >>>> +     void *to;
> >>>> +
> >>>> +     BUG_ON(pte_page(*pte) != walk->reuse_page);
> >>>> +
> >>>> +     page = list_first_entry(walk->vmemmap_pages, struct page, lru);
> >>>> +     list_del(&page->lru);
> >>>> +     to = page_to_virt(page);
> >>>> +     copy_page(to, (void *)walk->reuse_addr);
> >>>> +
> >>>> +     set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
> >>>> +}
> >>>> +
> >>>> +static void alloc_vmemmap_page_list(struct list_head *list,
> >>>> +                                 unsigned long start, unsigned long end)
> >>>> +{
> >>>> +     unsigned long addr;
> >>>> +
> >>>> +     for (addr = start; addr < end; addr += PAGE_SIZE) {
> >>>> +             struct page *page;
> >>>> +             int nid = page_to_nid((const void *)addr);
> >>>> +
> >>>> +retry:
> >>>> +             page = alloc_pages_node(nid, GFP_VMEMMAP_PAGE, 0);
> >>>> +             if (unlikely(!page)) {
> >>>> +                     msleep(100);
> >>>> +                     /*
> >>>> +                      * We should retry infinitely, because we cannot
> >>>> +                      * handle allocation failures. Once we allocate
> >>>> +                      * vmemmap pages successfully, then we can free
> >>>> +                      * a HugeTLB page.
> >>>> +                      */
> >>>> +                     goto retry;
> >>>
> >>> Ugh, I don't think this will work, there's no guarantee that we'll ever
> >>> succeed and now we can't free a 2MB hugepage because we cannot allocate a
> >>> 4KB page.  We absolutely have to ensure we make forward progress here.
> >>
> >> This can trigger a OOM when there is no memory and kill someone to release
> >> some memory. Right?
> >>
> >>>
> >>> We're going to be freeing the hugetlb page after this succeeeds, can we
> >>> not use part of the hugetlb page that we're freeing for this memory
> >>> instead?
> >>
> >> It seems a good idea. We can try to allocate memory firstly, if successful,
> >> just use the new page to remap (it can reduce memory fragmentation).
> >> If not, we can use part of the hugetlb page to remap. What's your opinion
> >> about this?
> >
> > If the HugeTLB page is a gigantic page which is allocated from
> > CMA. In this case, we cannot use part of the hugetlb page to remap.
> > Right?
>
> Right; and I don't think the "reuse part of a huge page as vmemmap while
> freeing, while that part itself might not have a proper vmemmap yet (or
> might cover itself now)" is particularly straight forward. Maybe I'm
> wrong :)
>
> Also, watch out for huge pages on ZONE_MOVABLE, in that case you also
> shouldn't allocate the vmemmap from there ...

Yeah, you are right. So I tend to trigger OOM to kill other processes to
reclaim some memory when we allocate memory fails.

>
> --
> Thanks,
>
> David / dhildenb
>