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Subject[PATCH v20 06/20] mm/thp: narrow lru locking
Date
lru_lock and page cache xa_lock have no obvious reason to be taken
one way round or the other: until now, lru_lock has been taken before
page cache xa_lock, when splitting a THP; but nothing else takes them
together. Reverse that ordering: let's narrow the lru locking - but
leave local_irq_disable to block interrupts throughout, like before.

Hugh Dickins point: split_huge_page_to_list() was already silly, to be
using the _irqsave variant: it's just been taking sleeping locks, so
would already be broken if entered with interrupts enabled. So we
can save passing flags argument down to __split_huge_page().

Why change the lock ordering here? That was hard to decide. One reason:
when this series reaches per-memcg lru locking, it relies on the THP's
memcg to be stable when taking the lru_lock: that is now done after the
THP's refcount has been frozen, which ensures page memcg cannot change.

Another reason: previously, lock_page_memcg()'s move_lock was presumed
to nest inside lru_lock; but now lru_lock must nest inside (page cache
lock inside) move_lock, so it becomes possible to use lock_page_memcg()
to stabilize page memcg before taking its lru_lock. That is not the
mechanism used in this series, but it is an option we want to keep open.

[Hugh Dickins: rewrite commit log]
Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
---
mm/huge_memory.c | 25 +++++++++++++------------
1 file changed, 13 insertions(+), 12 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 4b72dd7b8b34..5fa890e26975 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2433,7 +2433,7 @@ static void __split_huge_page_tail(struct page *head, int tail,
}

static void __split_huge_page(struct page *page, struct list_head *list,
- pgoff_t end, unsigned long flags)
+ pgoff_t end)
{
struct page *head = compound_head(page);
pg_data_t *pgdat = page_pgdat(head);
@@ -2443,8 +2443,6 @@ static void __split_huge_page(struct page *page, struct list_head *list,
unsigned int nr = thp_nr_pages(head);
int i;

- lruvec = mem_cgroup_page_lruvec(head, pgdat);
-
/* complete memcg works before add pages to LRU */
mem_cgroup_split_huge_fixup(head);

@@ -2456,6 +2454,11 @@ static void __split_huge_page(struct page *page, struct list_head *list,
xa_lock(&swap_cache->i_pages);
}

+ /* prevent PageLRU to go away from under us, and freeze lru stats */
+ spin_lock(&pgdat->lru_lock);
+
+ lruvec = mem_cgroup_page_lruvec(head, pgdat);
+
for (i = nr - 1; i >= 1; i--) {
__split_huge_page_tail(head, i, lruvec, list);
/* Some pages can be beyond i_size: drop them from page cache */
@@ -2475,6 +2478,8 @@ static void __split_huge_page(struct page *page, struct list_head *list,
}

ClearPageCompound(head);
+ spin_unlock(&pgdat->lru_lock);
+ /* Caller disabled irqs, so they are still disabled here */

split_page_owner(head, nr);

@@ -2492,8 +2497,7 @@ static void __split_huge_page(struct page *page, struct list_head *list,
page_ref_add(head, 2);
xa_unlock(&head->mapping->i_pages);
}
-
- spin_unlock_irqrestore(&pgdat->lru_lock, flags);
+ local_irq_enable();

remap_page(head, nr);

@@ -2639,12 +2643,10 @@ bool can_split_huge_page(struct page *page, int *pextra_pins)
int split_huge_page_to_list(struct page *page, struct list_head *list)
{
struct page *head = compound_head(page);
- struct pglist_data *pgdata = NODE_DATA(page_to_nid(head));
struct deferred_split *ds_queue = get_deferred_split_queue(head);
struct anon_vma *anon_vma = NULL;
struct address_space *mapping = NULL;
int count, mapcount, extra_pins, ret;
- unsigned long flags;
pgoff_t end;

VM_BUG_ON_PAGE(is_huge_zero_page(head), head);
@@ -2705,9 +2707,8 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
unmap_page(head);
VM_BUG_ON_PAGE(compound_mapcount(head), head);

- /* prevent PageLRU to go away from under us, and freeze lru stats */
- spin_lock_irqsave(&pgdata->lru_lock, flags);
-
+ /* block interrupt reentry in xa_lock and spinlock */
+ local_irq_disable();
if (mapping) {
XA_STATE(xas, &mapping->i_pages, page_index(head));

@@ -2737,7 +2738,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
__dec_node_page_state(head, NR_FILE_THPS);
}

- __split_huge_page(page, list, end, flags);
+ __split_huge_page(page, list, end);
ret = 0;
} else {
if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
@@ -2751,7 +2752,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
spin_unlock(&ds_queue->split_queue_lock);
fail: if (mapping)
xa_unlock(&mapping->i_pages);
- spin_unlock_irqrestore(&pgdata->lru_lock, flags);
+ local_irq_enable();
remap_page(head, thp_nr_pages(head));
ret = -EBUSY;
}
--
1.8.3.1
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