Re: [External] Re: [PATCH v14 3/8] mm: hugetlb: free the vmemmap pages associated with each HugeTLB page

[Muchun Song <songmuchun@bytedance.com>]

On Fri, Feb 5, 2021 at 4:54 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Thu, Feb 04, 2021 at 11:50:38AM +0800, Muchun Song wrote:
> > Every HugeTLB has more than one struct page structure. We __know__ that
> > we only use the first 4(HUGETLB_CGROUP_MIN_ORDER) struct page structures
> > to store metadata associated with each HugeTLB.
> >
> > There are a lot of struct page structures associated with each HugeTLB
> > page. For tail pages, the value of compound_head is the same. So we can
> > reuse first page of tail page structures. We map the virtual addresses
> > of the remaining pages of tail page structures to the first tail page
> > struct, and then free these page frames. Therefore, we need to reserve
> > two pages as vmemmap areas.
> >
> > When we allocate a HugeTLB page from the buddy, we can free some vmemmap
> > pages associated with each HugeTLB page. It is more appropriate to do it
> > in the prep_new_huge_page().
> >
> > The 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.
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
>
> [...]
>
> > +void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> > +{
> > +     unsigned long vmemmap_addr = (unsigned long)head;
> > +     unsigned long vmemmap_end, vmemmap_reuse;
> > +
> > +     if (!free_vmemmap_pages_per_hpage(h))
> > +             return;
> > +
> > +     vmemmap_addr += RESERVE_VMEMMAP_SIZE;
> > +     vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
> > +     vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
> > +
> > +     /*
> > +      * Remap the vmemmap virtual address range [@vmemmap_addr, @vmemmap_end)
> > +      * to the page which @vmemmap_reuse is mapped to, then free the vmemmap
> > +      * pages which the range are mapped to.
>
> "then free the pages which the range [@vmemmap_addr, @vmemmap_end] is mapped to."
>
> I am not a native but sounds better to me.

Me too. But I believe you are right. :-)

>
> > +      */
> > +     vmemmap_remap_free(vmemmap_addr, vmemmap_end, vmemmap_reuse);
> > +}
> > diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
> > new file mode 100644
> > index 000000000000..6923f03534d5
> > --- /dev/null
> > +++ b/mm/hugetlb_vmemmap.h
>
> [...]
>
> > diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> > index 16183d85a7d5..50c1dc00b686 100644
> > --- a/mm/sparse-vmemmap.c
> > +++ b/mm/sparse-vmemmap.c
> > @@ -27,8 +27,215 @@
> >  #include <linux/spinlock.h>
> >  #include <linux/vmalloc.h>
> >  #include <linux/sched.h>
> > +#include <linux/pgtable.h>
> > +#include <linux/bootmem_info.h>
> > +
> >  #include <asm/dma.h>
> >  #include <asm/pgalloc.h>
> > +#include <asm/tlbflush.h>
> > +
> > +/**
> > + * vmemmap_remap_walk - walk vmemmap page table
> > + *
> > + * @remap_pte:               called for each non-empty PTE (lowest-level) entry.
>
> Well, we BUG_ON on empty PTE, so not sure that pointing out here is worth.
> It sounds like we do nothing when it's empty.
> Maybe:
>
> "called for each lowest-level entry (PTE)"

Thanks. I will update this.

>
> > + * @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.
> > + */
> > +struct vmemmap_remap_walk {
> > +     void (*remap_pte)(pte_t *pte, unsigned long addr,
> > +                       struct vmemmap_remap_walk *walk);
> > +     struct page *reuse_page;
> > +     unsigned long reuse_addr;
> > +     struct list_head *vmemmap_pages;
> > +};
> > +
> > +static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
> > +                           unsigned long end,
> > +                           struct vmemmap_remap_walk *walk)
> > +{
> > +     pte_t *pte;
> > +
> > +     pte = pte_offset_kernel(pmd, addr);
> > +
> > +     /*
> > +      * The reuse_page is found 'first' in table walk before we start
> > +      * remapping (which is calling @walk->remap_pte).
> > +      */
> > +     if (!walk->reuse_page) {
> > +             BUG_ON(pte_none(*pte) || walk->reuse_addr != addr);
>
> I would rather have them in separate lines:
> BUG_ON(pte_none(*pte));
> BUG_ON(walk->reuse_addr != addr));
>
> It helps when trying to figure out when we explode. One could dig in the
> registers, but let's make it easier to find out.

OK. Will do.

>
> > +
>
> [...]
>
>
> > +static void vmemmap_remap_range(unsigned long start, unsigned long end,
> > +                             struct vmemmap_remap_walk *walk)
> > +{
> > +     unsigned long addr = start;
> > +     unsigned long next;
> > +     pgd_t *pgd;
> > +
> > +     VM_BUG_ON(!IS_ALIGNED(start, PAGE_SIZE));
> > +     VM_BUG_ON(!IS_ALIGNED(end, PAGE_SIZE));
> > +
> > +     pgd = pgd_offset_k(addr);
> > +     do {
> > +             BUG_ON(pgd_none(*pgd));
> > +
> > +             next = pgd_addr_end(addr, end);
> > +             vmemmap_p4d_range(pgd, addr, next, walk);
> > +     } while (pgd++, addr = next, addr != end);
> > +
> > +     /*
> > +      * We do not change the mapping of the vmemmap virtual address range
> > +      * [@start, @start + PAGE_SIZE) which belongs to the reuse range.
> > +      * So we not need to flush the TLB.
> > +      */
> > +     flush_tlb_kernel_range(start + PAGE_SIZE, end);
>
> I find that comment a bit confusing. I would rather describe what are we
> flushing instead of what we are not.
>

OK. Will update it.

>
> > +}
> > +
> > +/*
> > + * Free a vmemmap page. A vmemmap page can be allocated from the memblock
> > + * allocator or buddy allocator. If the PG_reserved flag is set, it means
> > + * that it allocated from the memblock allocator, just free it via the
> > + * free_bootmem_page(). Otherwise, use __free_page().
> > + */
> > +static inline void free_vmemmap_page(struct page *page)
> > +{
> > +     if (PageReserved(page))
> > +             free_bootmem_page(page);
> > +     else
> > +             __free_page(page);
> > +}
> > +
> > +/* Free a list of the vmemmap pages */
> > +static void free_vmemmap_page_list(struct list_head *list)
> > +{
> > +     struct page *page, *next;
> > +
> > +     list_for_each_entry_safe(page, next, list, lru) {
> > +             list_del(&page->lru);
> > +             free_vmemmap_page(page);
> > +     }
> > +}
> > +
> > +static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
> > +                           struct vmemmap_remap_walk *walk)
> > +{
> > +     /*
> > +      * Remap the tail pages as read-only to catch illegal write operation
> > +      * to the tail pages.
> > +      */
> > +     pgprot_t pgprot = PAGE_KERNEL_RO;
> > +     pte_t entry = mk_pte(walk->reuse_page, pgprot);
> > +     struct page *page = pte_page(*pte);
> > +
> > +     list_add(&page->lru, walk->vmemmap_pages);
> > +     set_pte_at(&init_mm, addr, pte, entry);
> > +}
> > +
> > +/**
> > + * vmemmap_remap_free - remap the vmemmap virtual address range [@start, @end)
> > + *                   to the page which @reuse is mapped to, then free vmemmap
> > + *                   which the range are mapped to.
> > + * @start:   start address of the vmemmap virtual address range that we want
> > + *           to remap.
> > + * @end:     end address of the vmemmap virtual address range that we want to
> > + *           remap.
> > + * @reuse:   reuse address.
> > + *
> > + * Note: This function depends on vmemmap being base page mapped. Please make
> > + * sure that the architecture disables PMD mapping of vmemmap pages when calling
> > + * this function.
>
> Well, we do not really depend on the architecture to not map the vmemmap range
> with PMDs, right? IIUC, that is driven by your boot parameter (patch#5), which
> overrides whatever the architecture can do.

Right. I will rework the comment here.

>
> Functional changes look good to me, so with all the above fixes, you can add:
>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>
>

Very thanks.

>
> --
> Oscar Salvador
> SUSE L3