On Mon, 2019-09-30 at 14:51 -0700, John Hubbard wrote:

On 9/27/19 4:40 PM, Leonardo Bras wrote:

quoted

As decribed, gup_pgd_range is a lockless pagetable walk. So, in order to
monitor against THP split/collapse with the couting method, it's necessary

s/couting/counting/

Thanks, fixed for v5.

quoted

to bound it with {start,end}_lockless_pgtbl_walk.

There are dummy functions, so it is not going to add any overhead on archs
that don't use this method.

Signed-off-by: Leonardo Bras <redacted>
---
 mm/gup.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/gup.c b/mm/gup.c
index 98f13ab37bac..7105c829cf44 100644
--- a/mm/gup.c
+++ b/mm/gup.c

@@ -2325,6 +2325,7 @@ static bool gup_fast_permitted(unsigned long start, unsigned long end)
 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 			  struct page **pages)
 {
+	struct mm_struct *mm;

I don't think that this local variable adds any value, so let's not use it.
Similar point in a few other patches too.

It avoids 1 deference of current->mm, it's a little performance gain.

quoted

 	unsigned long len, end;
 	unsigned long flags;
 	int nr = 0;

@@ -2352,9 +2353,12 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 
 	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
 	    gup_fast_permitted(start, end)) {
+		mm = current->mm;
+		start_lockless_pgtbl_walk(mm);
 		local_irq_save(flags);
 		gup_pgd_range(start, end, write ? FOLL_WRITE : 0, pages, &nr);
 		local_irq_restore(flags);
+		end_lockless_pgtbl_walk(mm);
 	}
 
 	return nr;

@@ -2404,6 +2408,7 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
 			unsigned int gup_flags, struct page **pages)
 {
 	unsigned long addr, len, end;
+	struct mm_struct *mm;

Same here.

quoted

 	int nr = 0, ret = 0;
 
 	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM)))

@@ -2421,9 +2426,12 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
 
 	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
 	    gup_fast_permitted(start, end)) {
+		mm = current->mm;
+		start_lockless_pgtbl_walk(mm);

Minor: I'd like to rename this register_lockless_pgtable_walker().

quoted

 		local_irq_disable();
 		gup_pgd_range(addr, end, gup_flags, pages, &nr);
 		local_irq_enable();
+		end_lockless_pgtbl_walk(mm);

...and deregister_lockless_pgtable_walker().

I have no problem changing the name, but I don't register/deregister
are good terms for this. 

I would rather use start/finish, begin/end, and so on. Register sounds
like something more complicated than what we are trying to achieve
here. 

thanks,

Thank you!