Re: [External] Re: [PATCH v8 04/12] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page

[Muchun Song <songmuchun@bytedance.com>]

On Thu, Dec 10, 2020 at 10:43 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Thu, Dec 10, 2020 at 11:55:18AM +0800, Muchun Song wrote:
> > The free_vmemmap_pages_per_hpage() which indicate that how many vmemmap
> > pages associated with a HugeTLB page that can be freed to the buddy
> > allocator just returns zero now, because all infrastructure is not
> > ready. Once all the infrastructure is ready, we will rework this
> > function to support the feature.
>
> I would reword the above to:
>
> "free_vmemmap_pages_per_hpage(), which indicates how many vmemmap
>  pages associated with a HugeTLB page can be freed, returns zero for
>  now, which means the feature is disabled.
>  We will enable it once all the infrastructure is there."

Thanks for your suggestion.

>
>  Or something along those lines.
>
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>
> Overall this looks good to me, and it has seen a considerable
> simplification, which is good.
> Some nits/questions below:
>
>
> > +#define vmemmap_hpage_addr_end(addr, end)                             \
> > +({                                                                    \
> > +     unsigned long __boundary;                                        \
> > +     __boundary = ((addr) + VMEMMAP_HPAGE_SIZE) & VMEMMAP_HPAGE_MASK; \
> > +     (__boundary - 1 < (end) - 1) ? __boundary : (end);               \
> > +})
>
> Maybe add a little comment explaining what are you trying to get here.

OK. Will do.

>
> > +/*
> > + * Walk a vmemmap address to the pmd it maps.
> > + */
> > +static pmd_t *vmemmap_to_pmd(unsigned long addr)
> > +{
> > +     pgd_t *pgd;
> > +     p4d_t *p4d;
> > +     pud_t *pud;
> > +     pmd_t *pmd;
> > +
> > +     pgd = pgd_offset_k(addr);
> > +     if (pgd_none(*pgd))
> > +             return NULL;
> > +
> > +     p4d = p4d_offset(pgd, addr);
> > +     if (p4d_none(*p4d))
> > +             return NULL;
> > +
> > +     pud = pud_offset(p4d, addr);
> > +     if (pud_none(*pud))
> > +             return NULL;
> > +
> > +     pmd = pmd_offset(pud, addr);
> > +     if (pmd_none(*pmd))
> > +             return NULL;
> > +
> > +     return pmd;
> > +}
>
> I saw that some people suggested to put all the non-hugetlb vmemmap
> functions under sparsemem-vmemmap.c, which makes some sense if some
> feature is going to re-use this code somehow. (I am not sure if the
> recent patches that take advantage of this feature for ZONE_DEVICE needs
> something like this).
>
> I do not have a strong opinion on this though.

Yeah, I also thought about this. I prefer moving the common code to
the sparsemem-vmemmap.c. If more people agree with this, I can do
this in the next version. :)

>
> > +static void vmemmap_reuse_pte_range(struct page *reuse, pte_t *pte,
> > +                                 unsigned long start, unsigned long end,
> > +                                 struct list_head *vmemmap_pages)
> > +{
> > +     /*
> > +      * Make the tail pages are mapped with read-only to catch
> > +      * illegal write operation to the tail pages.
> > +      */
> > +     pgprot_t pgprot = PAGE_KERNEL_RO;
> > +     pte_t entry = mk_pte(reuse, pgprot);
> > +     unsigned long addr;
> > +
> > +     for (addr = start; addr < end; addr += PAGE_SIZE, pte++) {
> > +             struct page *page;
> > +
> > +             VM_BUG_ON(pte_none(*pte));
>
> If it is none, page will be NULL and we will crash in the list_add
> below?

Yeah, I think that here should be a BUG_ON.

>
> > +static void vmemmap_remap_range(unsigned long start, unsigned long end,
> > +                             struct list_head *vmemmap_pages)
> > +{
> > +     pmd_t *pmd;
> > +     unsigned long next, addr = start;
> > +     struct page *reuse = NULL;
> > +
> > +     VM_BUG_ON(!IS_ALIGNED(start, PAGE_SIZE));
> > +     VM_BUG_ON(!IS_ALIGNED(end, PAGE_SIZE));
> > +     VM_BUG_ON((start >> PUD_SHIFT) != (end >> PUD_SHIFT));
> This last VM_BUG_ON, is to see if both fall under the same PUD table?

Right.

>
> > +
> > +     pmd = vmemmap_to_pmd(addr);
> > +     BUG_ON(!pmd);
>
> Which is the criteria you followed to make this BUG_ON and VM_BUG_ON
> in the check from vmemmap_reuse_pte_range?

Indeed, I am somewhat confused. Should be unified. I should use
BUG_ON here and in vmemmap_reuse_pte_range.

>
> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun