Re: [PATCH 10/12] hugetlb: batch PMD split for bulk vmemmap dedup
From: Muchun Song
Date: Wed Aug 30 2023 - 14:48:01 EST
On 2023/8/26 03:04, Mike Kravetz wrote:
From: Joao Martins <joao.m.martins@xxxxxxxxxx>
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. This brings down from 14.2secs into 7.9secs a 1T hugetlb
allocation.
Rebased by Mike Kravetz
Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
mm/hugetlb_vmemmap.c | 94 ++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 90 insertions(+), 4 deletions(-)
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 500a118915ff..904a64fe5669 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -26,6 +26,7 @@
* @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 bulk operations
*/
struct vmemmap_remap_walk {
void (*remap_pte)(pte_t *pte, unsigned long addr,
@@ -34,9 +35,11 @@ struct vmemmap_remap_walk {
struct page *reuse_page;
unsigned long reuse_addr;
struct list_head *vmemmap_pages;
+#define VMEMMAP_REMAP_ONLY_SPLIT 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 bulk)
{
pmd_t __pmd;
int i;
@@ -79,7 +82,8 @@ 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 (!bulk)
+ flush_tlb_kernel_range(start, start + PMD_SIZE);
A little weird to me. @bulk is used to indicate whether the TLB
should be flushed, however, the flag name is VMEMMAP_REMAP_ONLY_SPLIT,
it seems to tell me @bulk (calculated from walk->flags &
VMEMMAP_REMAP_ONLY_SPLIT)
is a indicator to only split the huge pmd entry. For me, I think
it is better to introduce another flag like VMEMMAP_SPLIT_WITHOUT_FLUSH
to indicate whether TLB should be flushed.
} else {
pte_free_kernel(&init_mm, pgtable);
}
@@ -119,18 +123,28 @@ static int vmemmap_pmd_range(pud_t *pud, unsigned long addr,
unsigned long end,
struct vmemmap_remap_walk *walk)
{
+ bool bulk;
pmd_t *pmd;
unsigned long next;
+ bulk = walk->flags & VMEMMAP_REMAP_ONLY_SPLIT;
pmd = pmd_offset(pud, addr);
do {
int ret;
- ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK);
+ ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK, bulk);
if (ret)
return ret;
next = pmd_addr_end(addr, end);
+
+ /*
+ * We are only splitting, not remapping the hugetlb vmemmap
+ * pages.
+ */
+ if (bulk)
+ continue;
Actually, we don not need a flag to detect this situation, you could
use "!@walk->remap_pte" to determine whether we should go into the
next level traversal of the page table. ->remap_pte is used to traverse
the pte entry, so it make senses to continue to the next pmd entry if
it is NULL.
+
vmemmap_pte_range(pmd, addr, next, walk);
} while (pmd++, addr = next, addr != end);
@@ -197,7 +211,8 @@ 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->flags & VMEMMAP_REMAP_ONLY_SPLIT))
+ flush_tlb_kernel_range(start, end);
This could be:
if (walk->remap_pte)
flush_tlb_kernel_range(start, end);
return 0;
}
@@ -296,6 +311,48 @@ 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;
+ LIST_HEAD(vmemmap_pages);
Unused variable?
+ struct vmemmap_remap_walk walk = {
+ .flags = VMEMMAP_REMAP_ONLY_SPLIT,
+ };
+
+ /*
+ * In order to make remapping routine most efficient for the huge pages,
+ * the routine of vmemmap page table walking has the following rules
+ * (see more details from the vmemmap_pte_range()):
+ *
+ * - The range [@start, @end) and the range [@reuse, @reuse + PAGE_SIZE)
+ * should be continuous.
+ * - The @reuse address is part of the range [@reuse, @end) that we are
+ * walking which is passed to vmemmap_remap_range().
+ * - The @reuse address is the first in the complete range.
+ *
+ * So we need to make sure that @start and @reuse meet the above rules.
+ */
The comments are duplicated, something like:
/* See the comment in the vmemmap_remap_free(). */
is enough.
+ 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 +377,7 @@ 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 *)start);
gfp_t gfp_mask = GFP_KERNEL | __GFP_THISNODE | __GFP_NORETRY |
@@ -606,11 +664,39 @@ void hugetlb_vmemmap_optimize_bulk(const struct hstate *h, struct page *head,
__hugetlb_vmemmap_optimize(h, head, bulk_pages);
}
+void 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;
+
+ static_branch_inc(&hugetlb_optimize_vmemmap_key);
Why? hugetlb_optimize_vmemmap_key is used as a switch to let
page_fixed_fake_head works properly for the vmemmap-optimized
HugeTLB pages, however, this function only splits the huge pmd
entry without optimizing the vmemmap pages. So it is wrong to
increase the static_key.
Thanks.
+
+ vmemmap_end = vmemmap_start + hugetlb_vmemmap_size(h);
+ vmemmap_reuse = vmemmap_start;
+ vmemmap_start += HUGETLB_VMEMMAP_RESERVE_SIZE;
+
+ /*
+ * Remap the vmemmap virtual address range [@vmemmap_start, @vmemmap_end)
+ * to the page which @vmemmap_reuse is mapped to, then free the pages
+ * which the range [@vmemmap_start, @vmemmap_end] is mapped to.
+ */
+ if (vmemmap_remap_split(vmemmap_start, vmemmap_end, vmemmap_reuse))
+ static_branch_dec(&hugetlb_optimize_vmemmap_key);
+}
+
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)
+ hugetlb_vmemmap_split(h, &folio->page);
+
+ flush_tlb_kernel_range(0, TLB_FLUSH_ALL);
+
list_for_each_entry(folio, folio_list, lru)
hugetlb_vmemmap_optimize_bulk(h, &folio->page, &vmemmap_pages);