x86/mm: Pre-allocate p4d/pud pages for vmalloc area
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Message ID 20200626093450.27741-1-joro@8bytes.org
State New
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  • x86/mm: Pre-allocate p4d/pud pages for vmalloc area
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Commit Message

Joerg Roedel June 26, 2020, 9:34 a.m. UTC
From: Joerg Roedel <jroedel@suse.de>

Pre-allocate the page-table pages for the vmalloc area at the level
which needs synchronization on x86. This is P4D for 5-level and PUD
for 4-level paging.

Doing this at boot makes sure all page-tables in the system have these
pages already and do not need to be synchronized at runtime. The
runtime synchronizatin takes the pgd_lock and iterates over all
page-tables in the system, so it can take quite long and is better
avoided.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
 arch/x86/mm/init_64.c | 55 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)

Comments

Peter Zijlstra June 26, 2020, 11:07 a.m. UTC | #1
On Fri, Jun 26, 2020 at 11:34:50AM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.de>
> 
> Pre-allocate the page-table pages for the vmalloc area at the level
> which needs synchronization on x86. This is P4D for 5-level and PUD
> for 4-level paging.
> 
> Doing this at boot makes sure all page-tables in the system have these
> pages already and do not need to be synchronized at runtime. The
> runtime synchronizatin takes the pgd_lock and iterates over all
> page-tables in the system, so it can take quite long and is better
> avoided.
> 
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---

Can't we now remove arch_sync_kernel_mappings() from this same file?
Joerg Roedel June 26, 2020, 11:17 a.m. UTC | #2
On Fri, Jun 26, 2020 at 01:07:31PM +0200, Peter Zijlstra wrote:
> Can't we now remove arch_sync_kernel_mappings() from this same file?

Only if we panic on allocation failure.


	Joerg
Peter Zijlstra June 26, 2020, 11:31 a.m. UTC | #3
On Fri, Jun 26, 2020 at 01:17:11PM +0200, Joerg Roedel wrote:
> On Fri, Jun 26, 2020 at 01:07:31PM +0200, Peter Zijlstra wrote:
> > Can't we now remove arch_sync_kernel_mappings() from this same file?
> 
> Only if we panic on allocation failure.

I think we do that in plenty places already, so sure ;-)
Peter Zijlstra June 26, 2020, 11:32 a.m. UTC | #4
On Fri, Jun 26, 2020 at 01:31:01PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 26, 2020 at 01:17:11PM +0200, Joerg Roedel wrote:
> > On Fri, Jun 26, 2020 at 01:07:31PM +0200, Peter Zijlstra wrote:
> > > Can't we now remove arch_sync_kernel_mappings() from this same file?
> > 
> > Only if we panic on allocation failure.
> 
> I think we do that in plenty places already, so sure ;-)

That is, this is boot time only, right? clone() would return -ENOMEM, as
it's part of the normal page-table copy.
Joerg Roedel June 26, 2020, 11:44 a.m. UTC | #5
On Fri, Jun 26, 2020 at 01:32:15PM +0200, Peter Zijlstra wrote:
> That is, this is boot time only, right? clone() would return -ENOMEM, as
> it's part of the normal page-table copy.

Yes, the pre-allocation happens shortly after the buddy allocator took
over from bootmem. I don't quite get what clone() has to do with it.


	Joerg

Patch
diff mbox series

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index dbae185511cd..475a4008445b 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1238,6 +1238,59 @@  static void __init register_page_bootmem_info(void)
 #endif
 }
 
+/*
+ * Pre-allocates page-table pages for the vmalloc area in the kernel page-table.
+ * Only the level which needs to be synchronized between all page-tables is
+ * allocated because the synchronization can be expensive.
+ */
+static void __init preallocate_vmalloc_pages(void)
+{
+	unsigned long addr;
+	const char *lvl;
+	int count = 0;
+
+	for (addr = VMALLOC_START; addr <= VMALLOC_END; addr = ALIGN(addr + 1, PGDIR_SIZE)) {
+		pgd_t *pgd = pgd_offset_k(addr);
+		p4d_t *p4d;
+		pud_t *pud;
+
+		p4d = p4d_offset(pgd, addr);
+		if (p4d_none(*p4d)) {
+			/* Can only happen with 5-level paging */
+			p4d = p4d_alloc(&init_mm, pgd, addr);
+			if (!p4d) {
+				lvl = "p4d";
+				goto failed;
+			}
+			count += 1;
+		}
+
+		if (pgtable_l5_enabled())
+			continue;
+
+		pud = pud_offset(p4d, addr);
+		if (pud_none(*pud)) {
+			/* Ends up here only with 4-level paging */
+			pud = pud_alloc(&init_mm, p4d, addr);
+			if (!pud) {
+				lvl = "pud";
+				goto failed;
+			}
+			count += 1;
+		}
+	}
+
+	return;
+
+failed:
+
+	/*
+	 * A failure here is not fatal - If the pages can be allocated later it
+	 * will be synchronized to other page-tables.
+	 */
+	pr_err("Failed to pre-allocate %s pages for vmalloc area\n", lvl);
+}
+
 void __init mem_init(void)
 {
 	pci_iommu_alloc();
@@ -1261,6 +1314,8 @@  void __init mem_init(void)
 	if (get_gate_vma(&init_mm))
 		kclist_add(&kcore_vsyscall, (void *)VSYSCALL_ADDR, PAGE_SIZE, KCORE_USER);
 
+	preallocate_vmalloc_pages();
+
 	mem_init_print_info(NULL);
 }