[PATCH 02/11] x86/mm/hotplug: Remove pgd_list use from the memory hotplug code

Ingo Molnar
2015-09-22T06:24:29Z

The memory hotplug code uses sync_global_pgds() to synchronize updates
to the global (&init_mm) kernel PGD and the task PGDs. It does this
by iterating over the pgd_list - which list closely tracks task
creation/destruction via fork/clone.

But we want to remove this list, so that it does not have to be
maintained from fork()/exit(), so convert the memory hotplug code
to use the task list to iterate over all pgds in the system.

Also improve the comments a bit, to make this function easier
to understand.

Only lightly tested, as I don't have a memory hotplug setup.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman Long <Waiman.Long@hp.com>
Cc: linux-mm@kvack.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/mm/init_64.c | 38 ++++++++++++++++++++++++++------------
1 file changed, 26 insertions(+), 12 deletions(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 30564e2752d3..7129e7647a76 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -33,6 +33,7 @@
#include <linux/nmi.h>
#include <linux/gfp.h>
#include <linux/kcore.h>
+#include <linux/oom.h>

#include <asm/processor.h>
#include <asm/bios_ebda.h>
@@ -160,8 +161,8 @@ static int __init nonx32_setup(char *str)
__setup("noexec32=", nonx32_setup);

/*
- * When memory was added/removed make sure all the processes MM have
- * suitable PGD entries in the local PGD level page.
+ * When memory was added/removed make sure all the process MMs have
+ * matching PGD entries in the local PGD level page as well.
*/
void sync_global_pgds(unsigned long start, unsigned long end, int removed)
{
@@ -169,29 +170,40 @@ void sync_global_pgds(unsigned long start, unsigned long end, int removed)

for (address = start; address <= end; address += PGDIR_SIZE) {
const pgd_t *pgd_ref = pgd_offset_k(address);
- struct page *page;
+ struct task_struct *g;

/*
- * When it is called after memory hot remove, pgd_none()
- * returns true. In this case (removed == 1), we must clear
- * the PGD entries in the local PGD level page.
+ * When this function is called after memory hot remove,
+ * pgd_none() already returns true, but only the reference
+ * kernel PGD has been cleared, not the process PGDs.
+ *
+ * So clear the affected entries in every process PGD as well:
*/
if (pgd_none(*pgd_ref) && !removed)
continue;

+ rcu_read_lock(); /* Task list walk */
spin_lock(&pgd_lock);
- list_for_each_entry(page, &pgd_list, lru) {
+
+ for_each_process(g) {
+ struct task_struct *p;
+ struct mm_struct *mm;
pgd_t *pgd;
spinlock_t *pgt_lock;

- pgd = (pgd_t *)page_address(page) + pgd_index(address);
- /* the pgt_lock only for Xen */
- pgt_lock = &pgd_page_get_mm(page)->page_table_lock;
+ p = find_lock_task_mm(g);
+ if (!p)
+ continue;
+
+ mm = p->mm;
+ pgd = mm->pgd;
+
+ /* The pgt_lock is only used by Xen: */
+ pgt_lock = &mm->page_table_lock;
spin_lock(pgt_lock);

if (!pgd_none(*pgd_ref) && !pgd_none(*pgd))
- BUG_ON(pgd_page_vaddr(*pgd)
- != pgd_page_vaddr(*pgd_ref));
+ BUG_ON(pgd_page_vaddr(*pgd) != pgd_page_vaddr(*pgd_ref));

if (removed) {
if (pgd_none(*pgd_ref) && !pgd_none(*pgd))
@@ -202,8 +214,10 @@ void sync_global_pgds(unsigned long start, unsigned long end, int removed)
}

spin_unlock(pgt_lock);
+ task_unlock(p);
}
spin_unlock(&pgd_lock);
+ rcu_read_unlock();
}
}

--
2.1.4