Re: [Patch v3] mm: thp: grab the lock before manipulation defer list

From: Yang Shi
Date: Fri Jan 17 2020 - 14:17:52 EST



On 1/17/20 7:38 AM, Kirill A. Shutemov wrote:
On Fri, Jan 17, 2020 at 01:31:50AM -0800, David Rientjes wrote:
On Fri, 17 Jan 2020, Michal Hocko wrote:

On Thu 16-01-20 14:01:59, David Rientjes wrote:
On Thu, 16 Jan 2020, Kirill Tkhai wrote:

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c5b5f74cfd4d..6450bbe394e2 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5360,10 +5360,12 @@ static int mem_cgroup_move_account(struct page *page,
}
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
- if (compound && !list_empty(page_deferred_list(page))) {
+ if (compound) {
spin_lock(&from->deferred_split_queue.split_queue_lock);
- list_del_init(page_deferred_list(page));
- from->deferred_split_queue.split_queue_len--;
+ if (!list_empty(page_deferred_list(page))) {
+ list_del_init(page_deferred_list(page));
+ from->deferred_split_queue.split_queue_len--;
+ }
spin_unlock(&from->deferred_split_queue.split_queue_lock);
}
#endif
@@ -5377,11 +5379,13 @@ static int mem_cgroup_move_account(struct page *page,
page->mem_cgroup = to;
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
- if (compound && list_empty(page_deferred_list(page))) {
+ if (compound) {
spin_lock(&to->deferred_split_queue.split_queue_lock);
- list_add_tail(page_deferred_list(page),
- &to->deferred_split_queue.split_queue);
- to->deferred_split_queue.split_queue_len++;
+ if (list_empty(page_deferred_list(page))) {
+ list_add_tail(page_deferred_list(page),
+ &to->deferred_split_queue.split_queue);
+ to->deferred_split_queue.split_queue_len++;
+ }
spin_unlock(&to->deferred_split_queue.split_queue_lock);
}
#endif
The patch looks OK for me. But there is another question. I forget, why we unconditionally
add a page with empty deferred list to deferred_split_queue. Shouldn't we also check that
it was initially in the list? Something like:

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d4394ae4e5be..0be0136adaa6 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5289,6 +5289,7 @@ static int mem_cgroup_move_account(struct page *page,
struct pglist_data *pgdat;
unsigned long flags;
unsigned int nr_pages = compound ? hpage_nr_pages(page) : 1;
+ bool split = false;
int ret;
bool anon;
@@ -5346,6 +5347,7 @@ static int mem_cgroup_move_account(struct page *page,
if (!list_empty(page_deferred_list(page))) {
list_del_init(page_deferred_list(page));
from->deferred_split_queue.split_queue_len--;
+ split = true;
}
spin_unlock(&from->deferred_split_queue.split_queue_lock);
}
@@ -5360,7 +5362,7 @@ static int mem_cgroup_move_account(struct page *page,
page->mem_cgroup = to;
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
- if (compound) {
+ if (compound && split) {
spin_lock(&to->deferred_split_queue.split_queue_lock);
if (list_empty(page_deferred_list(page))) {
list_add_tail(page_deferred_list(page),

I think that's a good point, especially considering that the current code
appears to unconditionally place any compound page on the deferred split
queue of the destination memcg. The correct list that it should appear
on, I believe, depends on whether the pmd has been split for the process
being moved: note the MC_TARGET_PAGE caveat in
mem_cgroup_move_charge_pte_range() that does not move the charge for
compound pages with split pmds. So when mem_cgroup_move_account() is
called with compound == true, we're moving the charge of the entire
compound page: why would it appear on that memcg's deferred split queue?
I believe Kirill asked how do we know that the page should be actually
added to the deferred list just from the list_empty check. In other
words what if the page hasn't been split at all?

Right, and I don't think that it necessarily is and the second
conditional in Wei's patch will always succeed unless we have raced. That
patch is for a lock concern but I think Kirill's question has uncovered
something more interesting.

Kirill S would definitely be best to answer Kirill T's question, but from
my understanding when mem_cgroup_move_account() is called with
compound == true that we always have an intact pmd (we never migrate
partial page charges for pages on the deferred split queue with the
current charge migration implementation) and thus the underlying page is
not eligible to be split and shouldn't be on the deferred split queue.

In other words, a page being on the deferred split queue for a memcg
should only happen when it is charged to that memcg. (This wasn't the
case when we only had per-node split queues.) I think that's currently
broken in mem_cgroup_move_account() before Wei's patch.
Right. It's broken indeed.

Hmm... Yes, definitely. I wasn't realized this at the first place.


We are dealing with anon page here. And it cannot be on deferred list as
long as it's mapped with PMD. We cannot get compound == true &&
!list_empty() on the (first) enter to the function. Any PMD-mapped page
will be put onto deferred by the function. This is wrong.

The fix is not obvious.

This comment got in mem_cgroup_move_charge_pte_range() my attention:

/*
* We can have a part of the split pmd here. Moving it
* can be done but it would be too convoluted so simply
* ignore such a partial THP and keep it in original
* memcg. There should be somebody mapping the head.
*/

That's exactly the case we care about: PTE-mapped THP that has to be split
under load. We don't move charge of them between memcgs and therefore we
should not move the page to different memcg.

I guess this will do the trick :P

It seems correct to me. In addition, memcg move charge just move PMD mapped THP, the THP should be never on the deferred split queue of "from" if it is PMD mapped, so actually we don't have to move it to the deferred split queue of "to".


diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c5b5f74cfd4d..e87ee4c10f6e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5359,14 +5359,6 @@ static int mem_cgroup_move_account(struct page *page,
__mod_lruvec_state(to_vec, NR_WRITEBACK, nr_pages);
}
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
- if (compound && !list_empty(page_deferred_list(page))) {
- spin_lock(&from->deferred_split_queue.split_queue_lock);
- list_del_init(page_deferred_list(page));
- from->deferred_split_queue.split_queue_len--;
- spin_unlock(&from->deferred_split_queue.split_queue_lock);
- }
-#endif
/*
* It is safe to change page->mem_cgroup here because the page
* is referenced, charged, and isolated - we can't race with
@@ -5376,16 +5368,6 @@ static int mem_cgroup_move_account(struct page *page,
/* caller should have done css_get */
page->mem_cgroup = to;
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
- if (compound && list_empty(page_deferred_list(page))) {
- spin_lock(&to->deferred_split_queue.split_queue_lock);
- list_add_tail(page_deferred_list(page),
- &to->deferred_split_queue.split_queue);
- to->deferred_split_queue.split_queue_len++;
- spin_unlock(&to->deferred_split_queue.split_queue_lock);
- }
-#endif
-
spin_unlock_irqrestore(&from->move_lock, flags);
ret = 0;