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

[David Hildenbrand <david@redhat.com>]

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 ...

-- 
Thanks,

David / dhildenb