.. SPDX-License-Identifier: GPL-2.0
-==================================
-Free some vmemmap pages of HugeTLB
-==================================
+=========================================
+A vmemmap diet for HugeTLB and Device DAX
+=========================================
+
+HugeTLB
+=======
The struct page structures (page structs) are used to describe a physical
page frame. By default, there is a one-to-one mapping from a page frame to
more than one struct page struct with PG_head (e.g. 8 per 2 MB HugeTLB page)
associated with each HugeTLB page. The compound_head() can handle this
correctly (more details refer to the comment above compound_head()).
+
+Device DAX
+==========
+
+The device-dax interface uses the same tail deduplication technique explained
+in the previous chapter, except when used with the vmemmap in
+the device (altmap).
+
+The following page sizes are supported in DAX: PAGE_SIZE (4K on x86_64),
+PMD_SIZE (2M on x86_64) and PUD_SIZE (1G on x86_64).
+
+The differences with HugeTLB are relatively minor.
+
+It only use 3 page structs for storing all information as opposed
+to 4 on HugeTLB pages.
+
+There's no remapping of vmemmap given that device-dax memory is not part of
+System RAM ranges initialized at boot. Thus the tail page deduplication
+happens at a later stage when we populate the sections. HugeTLB reuses the
+the head vmemmap page representing, whereas device-dax reuses the tail
+vmemmap page. This results in only half of the savings compared to HugeTLB.
+
+Deduplicated tail pages are not mapped read-only.
+
+Here's how things look like on device-dax after the sections are populated::
+
+ +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
+ | | | 0 | -------------> | 0 |
+ | | +-----------+ +-----------+
+ | | | 1 | -------------> | 1 |
+ | | +-----------+ +-----------+
+ | | | 2 | ----------------^ ^ ^ ^ ^ ^
+ | | +-----------+ | | | | |
+ | | | 3 | ------------------+ | | | |
+ | | +-----------+ | | | |
+ | | | 4 | --------------------+ | | |
+ | PMD | +-----------+ | | |
+ | level | | 5 | ----------------------+ | |
+ | mapping | +-----------+ | |
+ | | | 6 | ------------------------+ |
+ | | +-----------+ |
+ | | | 7 | --------------------------+
+ | | +-----------+
+ | |
+ | |
+ | |
+ +-----------+
}
pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, int node,
- struct vmem_altmap *altmap)
+ struct vmem_altmap *altmap,
+ struct page *reuse)
{
pte_t *pte = pte_offset_kernel(pmd, addr);
if (pte_none(*pte)) {
pte_t entry;
void *p;
- p = vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap);
- if (!p)
- return NULL;
+ if (!reuse) {
+ p = vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap);
+ if (!p)
+ return NULL;
+ } else {
+ /*
+ * When a PTE/PMD entry is freed from the init_mm
+ * there's a a free_pages() call to this page allocated
+ * above. Thus this get_page() is paired with the
+ * put_page_testzero() on the freeing path.
+ * This can only called by certain ZONE_DEVICE path,
+ * and through vmemmap_populate_compound_pages() when
+ * slab is available.
+ */
+ get_page(reuse);
+ p = page_to_virt(reuse);
+ }
entry = pfn_pte(__pa(p) >> PAGE_SHIFT, PAGE_KERNEL);
set_pte_at(&init_mm, addr, pte, entry);
}
}
static pte_t * __meminit vmemmap_populate_address(unsigned long addr, int node,
- struct vmem_altmap *altmap)
+ struct vmem_altmap *altmap,
+ struct page *reuse)
{
pgd_t *pgd;
p4d_t *p4d;
pmd = vmemmap_pmd_populate(pud, addr, node);
if (!pmd)
return NULL;
- pte = vmemmap_pte_populate(pmd, addr, node, altmap);
+ pte = vmemmap_pte_populate(pmd, addr, node, altmap, reuse);
if (!pte)
return NULL;
vmemmap_verify(pte, node, addr, addr + PAGE_SIZE);
static int __meminit vmemmap_populate_range(unsigned long start,
unsigned long end, int node,
- struct vmem_altmap *altmap)
+ struct vmem_altmap *altmap,
+ struct page *reuse)
{
unsigned long addr = start;
pte_t *pte;
for (; addr < end; addr += PAGE_SIZE) {
- pte = vmemmap_populate_address(addr, node, altmap);
+ pte = vmemmap_populate_address(addr, node, altmap, reuse);
if (!pte)
return -ENOMEM;
}
int __meminit vmemmap_populate_basepages(unsigned long start, unsigned long end,
int node, struct vmem_altmap *altmap)
{
- return vmemmap_populate_range(start, end, node, altmap);
+ return vmemmap_populate_range(start, end, node, altmap, NULL);
+}
+
+/*
+ * For compound pages bigger than section size (e.g. x86 1G compound
+ * pages with 2M subsection size) fill the rest of sections as tail
+ * pages.
+ *
+ * Note that memremap_pages() resets @nr_range value and will increment
+ * it after each range successful onlining. Thus the value or @nr_range
+ * at section memmap populate corresponds to the in-progress range
+ * being onlined here.
+ */
+static bool __meminit reuse_compound_section(unsigned long start_pfn,
+ struct dev_pagemap *pgmap)
+{
+ unsigned long nr_pages = pgmap_vmemmap_nr(pgmap);
+ unsigned long offset = start_pfn -
+ PHYS_PFN(pgmap->ranges[pgmap->nr_range].start);
+
+ return !IS_ALIGNED(offset, nr_pages) && nr_pages > PAGES_PER_SUBSECTION;
+}
+
+static pte_t * __meminit compound_section_tail_page(unsigned long addr)
+{
+ pte_t *pte;
+
+ addr -= PAGE_SIZE;
+
+ /*
+ * Assuming sections are populated sequentially, the previous section's
+ * page data can be reused.
+ */
+ pte = pte_offset_kernel(pmd_off_k(addr), addr);
+ if (!pte)
+ return NULL;
+
+ return pte;
+}
+
+static int __meminit vmemmap_populate_compound_pages(unsigned long start_pfn,
+ unsigned long start,
+ unsigned long end, int node,
+ struct dev_pagemap *pgmap)
+{
+ unsigned long size, addr;
+ pte_t *pte;
+ int rc;
+
+ if (reuse_compound_section(start_pfn, pgmap)) {
+ pte = compound_section_tail_page(start);
+ if (!pte)
+ return -ENOMEM;
+
+ /*
+ * Reuse the page that was populated in the prior iteration
+ * with just tail struct pages.
+ */
+ return vmemmap_populate_range(start, end, node, NULL,
+ pte_page(*pte));
+ }
+
+ size = min(end - start, pgmap_vmemmap_nr(pgmap) * sizeof(struct page));
+ for (addr = start; addr < end; addr += size) {
+ unsigned long next = addr, last = addr + size;
+
+ /* Populate the head page vmemmap page */
+ pte = vmemmap_populate_address(addr, node, NULL, NULL);
+ if (!pte)
+ return -ENOMEM;
+
+ /* Populate the tail pages vmemmap page */
+ next = addr + PAGE_SIZE;
+ pte = vmemmap_populate_address(next, node, NULL, NULL);
+ if (!pte)
+ return -ENOMEM;
+
+ /*
+ * Reuse the previous page for the rest of tail pages
+ * See layout diagram in Documentation/vm/vmemmap_dedup.rst
+ */
+ next += PAGE_SIZE;
+ rc = vmemmap_populate_range(next, last, node, NULL,
+ pte_page(*pte));
+ if (rc)
+ return -ENOMEM;
+ }
+
+ return 0;
}
struct page * __meminit __populate_section_memmap(unsigned long pfn,
{
unsigned long start = (unsigned long) pfn_to_page(pfn);
unsigned long end = start + nr_pages * sizeof(struct page);
+ int r;
if (WARN_ON_ONCE(!IS_ALIGNED(pfn, PAGES_PER_SUBSECTION) ||
!IS_ALIGNED(nr_pages, PAGES_PER_SUBSECTION)))
return NULL;
- if (vmemmap_populate(start, end, nid, altmap))
+ if (is_power_of_2(sizeof(struct page)) &&
+ pgmap && pgmap_vmemmap_nr(pgmap) > 1 && !altmap)
+ r = vmemmap_populate_compound_pages(pfn, start, end, nid, pgmap);
+ else
+ r = vmemmap_populate(start, end, nid, altmap);
+
+ if (r < 0)
return NULL;
return pfn_to_page(pfn);
projects / users / hch / configfs.git / commitdiff