[PATCH 1/1] mm/mmu_gather: limit tlb batch count and add schedule point in tlb_batch_pages_flush

[Jianxing Wang <wangjianxing@xxxxxxxxxxx>]

free a large list of pages maybe cause rcu_sched starved on
non-preemptible kernels. howerver free_unref_page_list maybe can't
cond_resched as it maybe called in interrupt or atomic context,
especially can't detect atomic context in CONFIG_PREEMPTION=n.

tlb flush batch count depends on PAGE_SIZE, it's too large if
PAGE_SIZE > 4K, here limit max batch size with 4K.
And add schedule point in tlb_batch_pages_flush.

rcu: rcu_sched kthread starved for 5359 jiffies! g454793 f0x0
RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=19
[...]
Call Trace:
   free_unref_page_list+0x19c/0x270
   release_pages+0x3cc/0x498
   tlb_flush_mmu_free+0x44/0x70
   zap_pte_range+0x450/0x738
   unmap_page_range+0x108/0x240
   unmap_vmas+0x74/0xf0
   unmap_region+0xb0/0x120
   do_munmap+0x264/0x438
   vm_munmap+0x58/0xa0
   sys_munmap+0x10/0x20
   syscall_common+0x24/0x38

Signed-off-by: Jianxing Wang <wangjianxing@xxxxxxxxxxx>
---
 include/asm-generic/tlb.h | 7 ++++++-
 mm/mmu_gather.c           | 7 +++++--
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index 2c68a545ffa7..47c7f93ca695 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -230,8 +230,13 @@ struct mmu_gather_batch {
 	struct page		*pages[0];
 };
 
+#if PAGE_SIZE > 4096UL
+#define MAX_GATHER_BATCH_SZ	4096
+#else
+#define MAX_GATHER_BATCH_SZ	PAGE_SIZE
+#endif
 #define MAX_GATHER_BATCH	\
-	((PAGE_SIZE - sizeof(struct mmu_gather_batch)) / sizeof(void *))
+	((MAX_GATHER_BATCH_SZ - sizeof(struct mmu_gather_batch)) / sizeof(void *))
 
 /*
  * Limit the maximum number of mmu_gather batches to reduce a risk of soft
diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
index afb7185ffdc4..f2c105810b3f 100644
--- a/mm/mmu_gather.c
+++ b/mm/mmu_gather.c
@@ -8,6 +8,7 @@
 #include <linux/rcupdate.h>
 #include <linux/smp.h>
 #include <linux/swap.h>
+#include <linux/slab.h>
 
 #include <asm/pgalloc.h>
 #include <asm/tlb.h>
@@ -27,7 +28,7 @@ static bool tlb_next_batch(struct mmu_gather *tlb)
 	if (tlb->batch_count == MAX_GATHER_BATCH_COUNT)
 		return false;
 
-	batch = (void *)__get_free_pages(GFP_NOWAIT | __GFP_NOWARN, 0);
+	batch = kmalloc(MAX_GATHER_BATCH_SZ, GFP_NOWAIT | __GFP_NOWARN);
 	if (!batch)
 		return false;
 
@@ -49,6 +50,8 @@ static void tlb_batch_pages_flush(struct mmu_gather *tlb)
 	for (batch = &tlb->local; batch && batch->nr; batch = batch->next) {
 		free_pages_and_swap_cache(batch->pages, batch->nr);
 		batch->nr = 0;
+
+		cond_resched();
 	}
 	tlb->active = &tlb->local;
 }
@@ -59,7 +62,7 @@ static void tlb_batch_list_free(struct mmu_gather *tlb)
 
 	for (batch = tlb->local.next; batch; batch = next) {
 		next = batch->next;
-		free_pages((unsigned long)batch, 0);
+		kfree(batch);
 	}
 	tlb->local.next = NULL;
 }
-- 
2.31.1