Re: [PATCH v4 03/11] mm: memcontrol: make lruvec lock safe when LRU pages are reparented
From: Waiman Long
Date: Wed May 25 2022 - 11:00:20 EST
On 5/25/22 06:20, Muchun Song wrote:
On Tue, May 24, 2022 at 03:23:11PM -0400, Waiman Long wrote:
On 5/24/22 02:05, Muchun Song wrote:
The diagram below shows how to make the folio lruvec lock safe when LRU
pages are reparented.
folio_lruvec_lock(folio)
retry:
lruvec = folio_lruvec(folio);
// The folio is reparented at this time.
spin_lock(&lruvec->lru_lock);
if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio)))
// Acquired the wrong lruvec lock and need to retry.
// Because this folio is on the parent memcg lruvec list.
goto retry;
// If we reach here, it means that folio_memcg(folio) is stable.
memcg_reparent_objcgs(memcg)
// lruvec belongs to memcg and lruvec_parent belongs to parent memcg.
spin_lock(&lruvec->lru_lock);
spin_lock(&lruvec_parent->lru_lock);
// Move all the pages from the lruvec list to the parent lruvec list.
spin_unlock(&lruvec_parent->lru_lock);
spin_unlock(&lruvec->lru_lock);
After we acquire the lruvec lock, we need to check whether the folio is
reparented. If so, we need to reacquire the new lruvec lock. On the
routine of the LRU pages reparenting, we will also acquire the lruvec
lock (will be implemented in the later patch). So folio_memcg() cannot
be changed when we hold the lruvec lock.
Since lruvec_memcg(lruvec) is always equal to folio_memcg(folio) after
we hold the lruvec lock, lruvec_memcg_debug() check is pointless. So
remove it.
This is a preparation for reparenting the LRU pages.
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
include/linux/memcontrol.h | 18 +++-----------
mm/compaction.c | 10 +++++++-
mm/memcontrol.c | 62 +++++++++++++++++++++++++++++-----------------
mm/swap.c | 4 +++
4 files changed, 55 insertions(+), 39 deletions(-)
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index ff1c1dd7e762..4042e4d21fe2 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -752,7 +752,9 @@ static inline struct lruvec *mem_cgroup_lruvec(struct mem_cgroup *memcg,
* folio_lruvec - return lruvec for isolating/putting an LRU folio
* @folio: Pointer to the folio.
*
- * This function relies on folio->mem_cgroup being stable.
+ * The lruvec can be changed to its parent lruvec when the page reparented.
+ * The caller need to recheck if it cares about this changes (just like
+ * folio_lruvec_lock() does).
*/
static inline struct lruvec *folio_lruvec(struct folio *folio)
{
@@ -771,15 +773,6 @@ struct lruvec *folio_lruvec_lock_irq(struct folio *folio);
struct lruvec *folio_lruvec_lock_irqsave(struct folio *folio,
unsigned long *flags);
-#ifdef CONFIG_DEBUG_VM
-void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio);
-#else
-static inline
-void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio)
-{
-}
-#endif
-
static inline
struct mem_cgroup *mem_cgroup_from_css(struct cgroup_subsys_state *css){
return css ? container_of(css, struct mem_cgroup, css) : NULL;
@@ -1240,11 +1233,6 @@ static inline struct lruvec *folio_lruvec(struct folio *folio)
return &pgdat->__lruvec;
}
-static inline
-void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio)
-{
-}
-
static inline struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *memcg)
{
return NULL;
diff --git a/mm/compaction.c b/mm/compaction.c
index 817098817302..1692b17db781 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -515,6 +515,8 @@ compact_folio_lruvec_lock_irqsave(struct folio *folio, unsigned long *flags,
{
struct lruvec *lruvec;
+ rcu_read_lock();
+retry:
lruvec = folio_lruvec(folio);
/* Track if the lock is contended in async mode */
@@ -527,7 +529,13 @@ compact_folio_lruvec_lock_irqsave(struct folio *folio, unsigned long *flags,
spin_lock_irqsave(&lruvec->lru_lock, *flags);
out:
- lruvec_memcg_debug(lruvec, folio);
+ if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) {
+ spin_unlock_irqrestore(&lruvec->lru_lock, *flags);
+ goto retry;
+ }
+
+ /* See the comments in folio_lruvec_lock(). */
+ rcu_read_unlock();
return lruvec;
}
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6de0d3e53eb1..b38a77f6696f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1199,23 +1199,6 @@ int mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
return ret;
}
-#ifdef CONFIG_DEBUG_VM
-void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio)
-{
- struct mem_cgroup *memcg;
-
- if (mem_cgroup_disabled())
- return;
-
- memcg = folio_memcg(folio);
-
- if (!memcg)
- VM_BUG_ON_FOLIO(lruvec_memcg(lruvec) != root_mem_cgroup, folio);
- else
- VM_BUG_ON_FOLIO(lruvec_memcg(lruvec) != memcg, folio);
-}
-#endif
-
/**
* folio_lruvec_lock - Lock the lruvec for a folio.
* @folio: Pointer to the folio.
@@ -1230,10 +1213,23 @@ void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio)
*/
struct lruvec *folio_lruvec_lock(struct folio *folio)
{
- struct lruvec *lruvec = folio_lruvec(folio);
+ struct lruvec *lruvec;
+ rcu_read_lock();
+retry:
+ lruvec = folio_lruvec(folio);
spin_lock(&lruvec->lru_lock);
- lruvec_memcg_debug(lruvec, folio);
+
+ if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) {
+ spin_unlock(&lruvec->lru_lock);
+ goto retry;
+ }
+
+ /*
+ * Preemption is disabled in the internal of spin_lock, which can serve
+ * as RCU read-side critical sections.
+ */
What is the point of this comment as preemption is not disabled for
PREEMPT_RT kernel?
I'm not familar with PREEMPT_RT kernel. At least you are right,
preemption is not disabled in this case, I think I should drop
this assumption.
Preemption is not disabled for PREEMPT_RT kernel but task migration to
another cpu is disabled. So access to per-cpu variables are safe. RCU
seems to have a special mode for PREEMPT_RT kernel but I am not familiar
with the detail.
Cheers,
Longman