Merge branch 'for-5.14-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/dennis/percpu

Pull percpu fix from Dennis Zhou:
"This is just a single change to fix percpu depopulation. The code
relied on depopulation code written specifically for the free path and
relied on vmalloc to do the tlb flush lazily. As we're modifying the
backing pages during the lifetime of a chunk, we need to also flush
the tlb accordingly.

Guenter Roeck reported this issue in [1] on mips. I believe we just
happen to be lucky given the much larger chunk sizes on x86 and
consequently less churning of this memory"

Link: https://lore.kernel.org/lkml/[e-mail]/ [1]

* 'for-5.14-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/dennis/percpu:
percpu: flush tlb in pcpu_reclaim_populated()

mm/percpu-km.c

···
 
 #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)
 {

mm/percpu-vm.c

···
  * 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.
  */
···
 	pcpu_pre_unmap_flush(chunk, page_start, page_end);
 
 	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);
 }

mm/percpu.c

···
  *
  * 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
···
 			       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);
···
 {
 	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
···
 		 * 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)
···
 
 			/* 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;
 			}
 
 			/*
···
 			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) {
+			spin_unlock_irq(&pcpu_lock);
+			pcpu_post_unmap_tlb_flush(chunk,
+						  freed_page_start,
+						  freed_page_end);
+			cond_resched();
+			spin_lock_irq(&pcpu_lock);
+		}
+
+		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]);
 	}
 }