Re: [External] Re: [PATCH v8 06/12] mm/hugetlb: Allocate the vmemmap pages associated with each HugeTLB page

[Muchun Song]

On Fri, Dec 11, 2020 at 5:35 PM Oscar Salvador <osalvador@xxxxxxx> wrote:
>
> On Thu, Dec 10, 2020 at 11:55:20AM +0800, Muchun Song wrote:
> > When we free a HugeTLB page to the buddy allocator, we should allocate the
> > vmemmap pages associated with it. We can do that in the __free_hugepage()
> "vmemmap pages that describe the range" would look better to me, but it is ok.

Thanks.

>
> > +#define GFP_VMEMMAP_PAGE             \
> > +     (GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_HIGH | __GFP_NOWARN)
> >
> >  #ifndef VMEMMAP_HPAGE_SHIFT
> >  #define VMEMMAP_HPAGE_SHIFT          HPAGE_SHIFT
> > @@ -197,6 +200,11 @@
> >       (__boundary - 1 < (end) - 1) ? __boundary : (end);               \
> >  })
> >
> > +typedef void (*vmemmap_remap_pte_func_t)(struct page *reuse, pte_t *pte,
> > +                                      unsigned long start, unsigned long end,
> > +                                      void *priv);
>
> Any reason to not have defined GFP_VMEMMAP_PAGE and the new typedef into
> hugetlb_vmemmap.h?

Because they can only be used in this hugetlb_vmemmap.c.

>
>
> > +static void vmemmap_restore_pte_range(struct page *reuse, pte_t *pte,
> > +                                   unsigned long start, unsigned long end,
> > +                                   void *priv)
> > +{
> > +     pgprot_t pgprot = PAGE_KERNEL;
> > +     void *from = page_to_virt(reuse);
> > +     unsigned long addr;
> > +     struct list_head *pages = priv;
> [...]
> > +
> > +             /*
> > +              * Make sure that any data that writes to the @to is made
> > +              * visible to the physical page.
> > +              */
> > +             flush_kernel_vmap_range(to, PAGE_SIZE);
>
> Correct me if I am wrong, but flush_kernel_vmap_range is a NOOP under arches which
> do not have ARCH_HAS_FLUSH_KERNEL_DCACHE_PAGE.
> Since we only enable support for x86_64, and x86_64 is one of those arches,
> could we remove this, and introduced later on in case we enable this feature
> on an arch that needs it?

OK. Will remove.

>
> I am not sure if you need to flush the range somehow, as you did in
> vmemmap_remap_range.
>
> > +retry:
> > +             page = alloc_page(GFP_VMEMMAP_PAGE);
> > +             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;
>
> I think this is the trickiest part.
> With 2MB HugeTLB pages we only need 6 pages, but with 1GB, the number of pages
> we need to allocate increases significantly (4088 pages IIRC).
> And you are using __GFP_HIGH, which will allow us to use more memory (by
> cutting down the watermark), but it might lead to putting the system
> on its knees wrt. memory.
> And yes, I know that once we allocate the 4088 pages, 1GB gets freed, but
> still.

Yeah, it is a problem. How about removing __GFP_HIGH only for
1GB HugeTLB page?

>
> I would like to hear Michal's thoughts on this one, but I wonder if it makes
> sense to not let 1GB-HugeTLB pages be freed.
>
> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun