[PATCH v8 6/8] hugetlb: batch PMD split for bulk vmemmap dedup

Mike Kravetz
2023-10-19T02:32:26Z

From: Joao Martins <joao.m.martins@oracle.com>

In an effort to minimize amount of TLB flushes, batch all PMD splits
belonging to a range of pages in order to perform only 1 (global) TLB
flush.

Add a flags field to the walker and pass whether it's a bulk allocation
or just a single page to decide to remap. First value
(VMEMMAP_SPLIT_NO_TLB_FLUSH) designates the request to not do the TLB
flush when we split the PMD.

Rebased and updated by Mike Kravetz

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
---
mm/hugetlb_vmemmap.c | 92 ++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 88 insertions(+), 4 deletions(-)

diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 4ac521e596db..10739e4285d5 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -27,6 +27,8 @@
* @reuse_addr: the virtual address of the @reuse_page page.
* @vmemmap_pages: the list head of the vmemmap pages that can be freed
* or is mapped from.
+ * @flags: used to modify behavior in vmemmap page table walking
+ * operations.
*/
struct vmemmap_remap_walk {
void (*remap_pte)(pte_t *pte, unsigned long addr,
@@ -35,9 +37,13 @@ struct vmemmap_remap_walk {
struct page *reuse_page;
unsigned long reuse_addr;
struct list_head *vmemmap_pages;
+
+/* Skip the TLB flush when we split the PMD */
+#define VMEMMAP_SPLIT_NO_TLB_FLUSH BIT(0)
+ unsigned long flags;
};

-static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
+static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start, bool flush)
{
pmd_t __pmd;
int i;
@@ -80,7 +86,8 @@ static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
/* Make pte visible before pmd. See comment in pmd_install(). */
smp_wmb();
pmd_populate_kernel(&init_mm, pmd, pgtable);
- flush_tlb_kernel_range(start, start + PMD_SIZE);
+ if (flush)
+ flush_tlb_kernel_range(start, start + PMD_SIZE);
} else {
pte_free_kernel(&init_mm, pgtable);
}
@@ -127,11 +134,20 @@ static int vmemmap_pmd_range(pud_t *pud, unsigned long addr,
do {
int ret;

- ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK);
+ ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK,
+ !(walk->flags & VMEMMAP_SPLIT_NO_TLB_FLUSH));
if (ret)
return ret;

next = pmd_addr_end(addr, end);
+
+ /*
+ * We are only splitting, not remapping the hugetlb vmemmap
+ * pages.
+ */
+ if (!walk->remap_pte)
+ continue;
+
vmemmap_pte_range(pmd, addr, next, walk);
} while (pmd++, addr = next, addr != end);

@@ -198,7 +214,8 @@ static int vmemmap_remap_range(unsigned long start, unsigned long end,
return ret;
} while (pgd++, addr = next, addr != end);

- flush_tlb_kernel_range(start, end);
+ if (walk->remap_pte)
+ flush_tlb_kernel_range(start, end);

return 0;
}
@@ -297,6 +314,36 @@ static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
}

+/**
+ * vmemmap_remap_split - split the vmemmap virtual address range [@start, @end)
+ * backing PMDs of the directmap into PTEs
+ * @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.
+ *
+ * Return: %0 on success, negative error code otherwise.
+ */
+static int vmemmap_remap_split(unsigned long start, unsigned long end,
+ unsigned long reuse)
+{
+ int ret;
+ struct vmemmap_remap_walk walk = {
+ .remap_pte = NULL,
+ .flags = VMEMMAP_SPLIT_NO_TLB_FLUSH,
+ };
+
+ /* See the comment in the vmemmap_remap_free(). */
+ BUG_ON(start - reuse != PAGE_SIZE);
+
+ mmap_read_lock(&init_mm);
+ ret = vmemmap_remap_range(reuse, end, &walk);
+ mmap_read_unlock(&init_mm);
+
+ return ret;
+}
+
/**
* vmemmap_remap_free - remap the vmemmap virtual address range [@start, @end)
* to the page which @reuse is mapped to, then free vmemmap
@@ -320,6 +367,7 @@ static int vmemmap_remap_free(unsigned long start, unsigned long end,
.remap_pte = vmemmap_remap_pte,
.reuse_addr = reuse,
.vmemmap_pages = vmemmap_pages,
+ .flags = 0,
};
int nid = page_to_nid((struct page *)reuse);
gfp_t gfp_mask = GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN;
@@ -368,6 +416,7 @@ static int vmemmap_remap_free(unsigned long start, unsigned long end,
.remap_pte = vmemmap_restore_pte,
.reuse_addr = reuse,
.vmemmap_pages = vmemmap_pages,
+ .flags = 0,
};

vmemmap_remap_range(reuse, end, &walk);
@@ -419,6 +468,7 @@ static int vmemmap_remap_alloc(unsigned long start, unsigned long end,
.remap_pte = vmemmap_restore_pte,
.reuse_addr = reuse,
.vmemmap_pages = &vmemmap_pages,
+ .flags = 0,
};

/* See the comment in the vmemmap_remap_free(). */
@@ -628,11 +678,45 @@ void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head)
free_vmemmap_page_list(&vmemmap_pages);
}

+static int hugetlb_vmemmap_split(const struct hstate *h, struct page *head)
+{
+ unsigned long vmemmap_start = (unsigned long)head, vmemmap_end;
+ unsigned long vmemmap_reuse;
+
+ if (!vmemmap_should_optimize(h, head))
+ return 0;
+
+ vmemmap_end = vmemmap_start + hugetlb_vmemmap_size(h);
+ vmemmap_reuse = vmemmap_start;
+ vmemmap_start += HUGETLB_VMEMMAP_RESERVE_SIZE;
+
+ /*
+ * Split PMDs on the vmemmap virtual address range [@vmemmap_start,
+ * @vmemmap_end]
+ */
+ return vmemmap_remap_split(vmemmap_start, vmemmap_end, vmemmap_reuse);
+}
+
void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head *folio_list)
{
struct folio *folio;
LIST_HEAD(vmemmap_pages);

+ list_for_each_entry(folio, folio_list, lru) {
+ int ret = hugetlb_vmemmap_split(h, &folio->page);
+
+ /*
+ * Spliting the PMD requires allocating a page, thus lets fail
+ * early once we encounter the first OOM. No point in retrying
+ * as it can be dynamically done on remap with the memory
+ * we get back from the vmemmap deduplication.
+ */
+ if (ret == -ENOMEM)
+ break;
+ }
+
+ flush_tlb_all();
+
list_for_each_entry(folio, folio_list, lru) {
int ret = __hugetlb_vmemmap_optimize(h, &folio->page,
&vmemmap_pages);
--
2.41.0