Re: [External] Re: [PATCH v10 03/11] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page

On Mon, Dec 21, 2020 at 5:11 PM Oscar Salvador [off-list ref] wrote:

On Thu, Dec 17, 2020 at 08:12:55PM +0800, Muchun Song wrote:

quoted

+static inline void free_bootmem_page(struct page *page)
+{
+     unsigned long magic = (unsigned long)page->freelist;
+
+     /*
+      * The reserve_bootmem_region sets the reserved flag on bootmem
+      * pages.
+      */
+     VM_WARN_ON(page_ref_count(page) != 2);
+
+     if (magic == SECTION_INFO || magic == MIX_SECTION_INFO)
+             put_page_bootmem(page);
+     else
+             VM_WARN_ON(1);

Ideally, I think we want to see what how the page looks since its state
is not what we expected, so maybe join both conditions and use dump_page().

Agree. Will do. Thanks.

quoted

+ * By removing redundant page structs for HugeTLB pages, memory can returned to

                                                                     ^^ be

Thanks.

quoted

+ * the buddy allocator for other uses.

[...]

quoted

+void free_huge_page_vmemmap(struct hstate *h, struct page *head)
+{
+     unsigned long vmemmap_addr = (unsigned long)head;
+
+     if (!free_vmemmap_pages_per_hpage(h))
+             return;
+
+     vmemmap_remap_free(vmemmap_addr + RESERVE_VMEMMAP_SIZE,
+                        free_vmemmap_pages_size_per_hpage(h));

I am not sure what others think, but I would like to see vmemmap_remap_free taking
three arguments: start, end, and reuse addr, e.g:

 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_MEMMAP_SIZE;
      vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
      vmemmap_reuse = vmemmap_addr - PAGE_SIZE;

      vmemmap_remap_free(vmemmap_addr, vmemmap_end, vmemmap_reuse);
 }

The reason for me to do this is to let the callers of vmemmap_remap_free decide
__what__ they want to remap.

More on this below.

quoted

+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);
+
+     if (walk->reuse_addr == addr) {
+             BUG_ON(pte_none(*pte));
+             walk->reuse_page = pte_page(*pte++);
+             addr += PAGE_SIZE;
+     }

Although it is quite obvious, a brief comment here pointing out what are we
doing and that this is meant to be set only once would be nice.

OK. Will do.

quoted

+static void vmemmap_remap_range(unsigned long start, unsigned long end,
+                             struct vmemmap_remap_walk *walk)
+{
+     unsigned long addr = start - PAGE_SIZE;
+     unsigned long next;
+     pgd_t *pgd;
+
+     VM_BUG_ON(!IS_ALIGNED(start, PAGE_SIZE));
+     VM_BUG_ON(!IS_ALIGNED(end, PAGE_SIZE));
+
+     walk->reuse_page = NULL;
+     walk->reuse_addr = addr;

With the change I suggested above, struct vmemmap_remap_walk should be
initialitzed at once in vmemmap_remap_free, so this should not longer be needed.

You are right.

(And btw, you do not need to set reuse_page to NULL, the way you init the struct
in vmemmap_remap_free makes sure to null any field you do not explicitly set).

quoted

+static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
+                           struct vmemmap_remap_walk *walk)
+{
+     /*
+      * Make the tail pages are mapped with read-only to catch
+      * illegal write operation to the tail pages.

        "Remap the tail pages as read-only to ..."

Thanks.

quoted

+      */
+     pgprot_t pgprot = PAGE_KERNEL_RO;
+     pte_t entry = mk_pte(walk->reuse_page, pgprot);
+     struct page *page;
+
+     page = pte_page(*pte);

 struct page *page = pte_page(*pte);

since you did the same for the other two.

Yeah. Will change to this.

quoted

+     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, start + size) to the page which
+ *                      [start - PAGE_SIZE, start) is mapped,
+ *                      then free vmemmap pages.
+ * @start:   start address of the vmemmap virtual address range
+ * @size:    size of the vmemmap virtual address range
+ */
+void vmemmap_remap_free(unsigned long start, unsigned long size)
+{
+     unsigned long end = start + size;
+     LIST_HEAD(vmemmap_pages);
+
+     struct vmemmap_remap_walk walk = {
+             .remap_pte      = vmemmap_remap_pte,
+             .vmemmap_pages  = &vmemmap_pages,
+     };

As stated above, this would become:

 void vmemmap_remap_free(unsigned long start, unsigned long end,
                         usigned long reuse)
 {
       LIST_HEAD(vmemmap_pages);
       struct vmemmap_remap_walk walk = {
               .reuse_addr = reuse,
               .remap_pte = vmemmap_remap_pte,
               .vmemmap_pages = &vmemmap_pages,
       };

  You might have had your reasons to do this way, but this looks more natural
  to me, with the plus that callers of vmemmap_remap_free can specify
  what they want to remap.

Should we add a BUG_ON in vmemmap_remap_free() for now?

        BUG_ON(reuse != start + PAGE_SIZE);

--
Oscar Salvador
SUSE L3

-- 
Yours,
Muchun