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 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?

>
> >
> > > +             }
> > > +             list_add_tail(&page->lru, list);
> > > +     }
> > > +}
> > > +
> > > +/**
> > > + * vmemmap_remap_alloc - remap the vmemmap virtual address range [@start, end)
> > > + *                    to the page which is from the @vmemmap_pages
> > > + *                    respectively.
> > > + * @start:   start address of the vmemmap virtual address range.
> > > + * @end:     end address of the vmemmap virtual address range.
> > > + * @reuse:   reuse address.
> > > + */
> > > +void vmemmap_remap_alloc(unsigned long start, unsigned long end,
> > > +                      unsigned long reuse)
> > > +{
> > > +     LIST_HEAD(vmemmap_pages);
> > > +     struct vmemmap_remap_walk walk = {
> > > +             .remap_pte      = vmemmap_restore_pte,
> > > +             .reuse_addr     = reuse,
> > > +             .vmemmap_pages  = &vmemmap_pages,
> > > +     };
> > > +
> > > +     might_sleep();
> > > +
> > > +     /* See the comment in the vmemmap_remap_free(). */
> > > +     BUG_ON(start - reuse != PAGE_SIZE);
> > > +
> > > +     alloc_vmemmap_page_list(&vmemmap_pages, start, end);
> > > +     vmemmap_remap_range(reuse, end, &walk);
> > > +}
> > > +
> > >  /*
> > >   * Allocate a block of memory to be used to back the virtual memory map
> > >   * or to back the page tables that are used to create the mapping.
> > > --
> > > 2.11.0
> > >
> > >