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Thu, 16 Jan 2020 12:35:01 +0300 Subject: Re: [Patch v3] mm: thp: grab the lock before manipulation defer list To: Wei Yang , hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com, akpm@linux-foundation.org, kirill.shutemov@linux.intel.com, yang.shi@linux.alibaba.com Cc: cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, alexander.duyck@gmail.com, rientjes@google.com, stable@vger.kernel.org References: <20200116013100.7679-1-richardw.yang@linux.intel.com> From: Kirill Tkhai Message-ID: <0bb34c4a-97c7-0b3c-cf43-8af6cf9c4396@virtuozzo.com> Date: Thu, 16 Jan 2020 12:35:00 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <20200116013100.7679-1-richardw.yang@linux.intel.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 16.01.2020 04:31, Wei Yang wrote: > As all the other places, we grab the lock before manipulate the defer list. > Current implementation may face a race condition. > > For example, the potential race would be: > > CPU1 CPU2 > mem_cgroup_move_account deferred_split_huge_page > list_empty > lock > list_empty > list_add_tail > unlock > lock > # list_empty might not hold anymore > list_add_tail > unlock > > When this sequence happens, the list_add_tail() in > mem_cgroup_move_account() corrupt the list since which is already been > added to some split_queue in split_huge_page_to_list(). > > Besides this, David Rientjes points out the split_queue_len would be in > a wrong state, which would be a significant issue for shrinkers. > > Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware") > > Signed-off-by: Wei Yang > Cc: [5.4+] > > --- > v3: > * remove all review/ack tag since rewrite the changelog > * use deferred_split_huge_page as the example of race > * add cc stable 5.4+ tag as suggested by David Rientjes > > v2: > * move check on compound outside suggested by Alexander > * an example of the race condition, suggested by Michal > --- > mm/memcontrol.c | 18 +++++++++++------- > 1 file changed, 11 insertions(+), 7 deletions(-) > > 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),