[PATCH] percpu: flush tlb in pcpu_reclaim_populated()

[Dennis Zhou]

Prior to "percpu: implement partial chunk depopulation",
pcpu_depopulate_chunk() was called only on the destruction path. This
meant the virtual address range was on its way back to vmalloc which
will handle flushing the tlbs for us.

However, with pcpu_reclaim_populated(), we are now calling
pcpu_depopulate_chunk() during the active lifecycle of a chunk.
Therefore, we need to flush the tlb as well otherwise we can end up
accessing the wrong page through an invalid tlb mapping as reported in
[1].

[1] https://lore.kernel.org/lkml/20210702191140.GA3166599@xxxxxxxxxxxx/

Fixes: f183324133ea ("percpu: implement partial chunk depopulation")
Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx>
Signed-off-by: Dennis Zhou <dennis@xxxxxxxxxx>
---
I think I'm happier with this. It does the same thing as [2] but moves
the flush to the caller so we can batch per chunk.

[2] https://lore.kernel.org/lkml/20210703040449.3213210-1-dennis@xxxxxxxxxx/

 mm/percpu-km.c |  6 ++++++
 mm/percpu-vm.c |  5 +++--
 mm/percpu.c    | 29 +++++++++++++++++++++++------
 3 files changed, 32 insertions(+), 8 deletions(-)

diff --git a/mm/percpu-km.c b/mm/percpu-km.c
index c9d529dc7651..fe31aa19db81 100644
--- a/mm/percpu-km.c
+++ b/mm/percpu-km.c
@@ -32,6 +32,12 @@
 
 #include <linux/log2.h>
 
+static void pcpu_post_unmap_tlb_flush(struct pcpu_chunk *chunk,
+				      int page_start, int page_end)
+{
+	/* nothing */
+}
+
 static int pcpu_populate_chunk(struct pcpu_chunk *chunk,
 			       int page_start, int page_end, gfp_t gfp)
 {
diff --git a/mm/percpu-vm.c b/mm/percpu-vm.c
index ee5d89fcd66f..2054c9213c43 100644
--- a/mm/percpu-vm.c
+++ b/mm/percpu-vm.c
@@ -303,6 +303,9 @@ static int pcpu_populate_chunk(struct pcpu_chunk *chunk,
  * For each cpu, depopulate and unmap pages [@page_start,@page_end)
  * from @chunk.
  *
+ * Caller is required to call pcpu_post_unmap_tlb_flush() if not returning the
+ * region back to vmalloc() which will lazily flush the tlb.
+ *
  * CONTEXT:
  * pcpu_alloc_mutex.
  */
@@ -324,8 +327,6 @@ static void pcpu_depopulate_chunk(struct pcpu_chunk *chunk,
 
 	pcpu_unmap_pages(chunk, pages, page_start, page_end);
 
-	/* no need to flush tlb, vmalloc will handle it lazily */
-
 	pcpu_free_pages(chunk, pages, page_start, page_end);
 }
 
diff --git a/mm/percpu.c b/mm/percpu.c
index b4cebeca4c0c..8d8efd668f76 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -1572,6 +1572,7 @@ static void pcpu_chunk_depopulated(struct pcpu_chunk *chunk,
  *
  * pcpu_populate_chunk		- populate the specified range of a chunk
  * pcpu_depopulate_chunk	- depopulate the specified range of a chunk
+ * pcpu_post_unmap_tlb_flush	- flush tlb for the specified range of a chunk
  * pcpu_create_chunk		- create a new chunk
  * pcpu_destroy_chunk		- destroy a chunk, always preceded by full depop
  * pcpu_addr_to_page		- translate address to physical address
@@ -1581,6 +1582,8 @@ static int pcpu_populate_chunk(struct pcpu_chunk *chunk,
 			       int page_start, int page_end, gfp_t gfp);
 static void pcpu_depopulate_chunk(struct pcpu_chunk *chunk,
 				  int page_start, int page_end);
+static void pcpu_post_unmap_tlb_flush(struct pcpu_chunk *chunk,
+				      int page_start, int page_end);
 static struct pcpu_chunk *pcpu_create_chunk(gfp_t gfp);
 static void pcpu_destroy_chunk(struct pcpu_chunk *chunk);
 static struct page *pcpu_addr_to_page(void *addr);
@@ -2137,11 +2140,12 @@ static void pcpu_reclaim_populated(void)
 {
 	struct pcpu_chunk *chunk;
 	struct pcpu_block_md *block;
+	int freed_page_start, freed_page_end;
 	int i, end;
+	bool reintegrate;
 
 	lockdep_assert_held(&pcpu_lock);
 
-restart:
 	/*
 	 * Once a chunk is isolated to the to_depopulate list, the chunk is no
 	 * longer discoverable to allocations whom may populate pages.  The only
@@ -2157,6 +2161,9 @@ static void pcpu_reclaim_populated(void)
 		 * Scan chunk's pages in the reverse order to keep populated
 		 * pages close to the beginning of the chunk.
 		 */
+		freed_page_start = chunk->nr_pages;
+		freed_page_end = 0;
+		reintegrate = false;
 		for (i = chunk->nr_pages - 1, end = -1; i >= 0; i--) {
 			/* no more work to do */
 			if (chunk->nr_empty_pop_pages == 0)
@@ -2164,8 +2171,8 @@ static void pcpu_reclaim_populated(void)
 
 			/* reintegrate chunk to prevent atomic alloc failures */
 			if (pcpu_nr_empty_pop_pages < PCPU_EMPTY_POP_PAGES_HIGH) {
-				pcpu_reintegrate_chunk(chunk);
-				goto restart;
+				reintegrate = true;
+				goto end_chunk;
 			}
 
 			/*
@@ -2194,16 +2201,26 @@ static void pcpu_reclaim_populated(void)
 			spin_lock_irq(&pcpu_lock);
 
 			pcpu_chunk_depopulated(chunk, i + 1, end + 1);
+			freed_page_start = min(freed_page_start, i + 1);
+			freed_page_end = max(freed_page_end, end + 1);
 
 			/* reset the range and continue */
 			end = -1;
 		}
 
-		if (chunk->free_bytes == pcpu_unit_size)
+end_chunk:
+		/* batch tlb flush per chunk to amortize cost */
+		if (freed_page_start < freed_page_end) {
+			pcpu_post_unmap_tlb_flush(chunk,
+						  freed_page_start,
+						  freed_page_end);
+		}
+
+		if (reintegrate || chunk->free_bytes == pcpu_unit_size)
 			pcpu_reintegrate_chunk(chunk);
 		else
-			list_move(&chunk->list,
-				  &pcpu_chunk_lists[pcpu_sidelined_slot]);
+			list_move_tail(&chunk->list,
+				       &pcpu_chunk_lists[pcpu_sidelined_slot]);
 	}
 }
 
-- 
2.32.0.93.g670b81a890-goog