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Message-ID: <20200117221859.GA29229@richard>
Date: Sat, 18 Jan 2020 06:18:59 +0800
From: Wei Yang <richardw.yang@...ux.intel.com>
To: Yang Shi <yang.shi@...ux.alibaba.com>
Cc: "Kirill A. Shutemov" <kirill@...temov.name>,
David Rientjes <rientjes@...gle.com>,
Michal Hocko <mhocko@...nel.org>,
Kirill Tkhai <ktkhai@...tuozzo.com>,
Wei Yang <richardw.yang@...ux.intel.com>, hannes@...xchg.org,
vdavydov.dev@...il.com, akpm@...ux-foundation.org,
kirill.shutemov@...ux.intel.com, cgroups@...r.kernel.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
alexander.duyck@...il.com, stable@...r.kernel.org
Subject: Re: [Patch v3] mm: thp: grab the lock before manipulation defer list
On Fri, Jan 17, 2020 at 11:17:38AM -0800, Yang Shi wrote:
>
>
>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".
>
Well, I got the point.
Since Kirill S found the correct solution, should I prepare v3 or Kirill will
send it?
>>
>> 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;
--
Wei Yang
Help you, Help me
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