diff --git a/memory/mozjemalloc/jemalloc.c b/memory/mozjemalloc/jemalloc.c
index 289c25e07d0b..6dd3c16e82fa 100644
--- a/memory/mozjemalloc/jemalloc.c
+++ b/memory/mozjemalloc/jemalloc.c
@@ -1385,7 +1385,7 @@ static void arena_chunk_init(arena_t *arena, arena_chunk_t *chunk);
 static void	arena_chunk_dealloc(arena_t *arena, arena_chunk_t *chunk);
 static arena_run_t *arena_run_alloc(arena_t *arena, arena_bin_t *bin,
     size_t size, bool large, bool zero);
-static void	arena_purge(arena_t *arena);
+static void	arena_purge(arena_t *arena, bool all);
 static void	arena_run_dalloc(arena_t *arena, arena_run_t *run, bool dirty);
 static void	arena_run_trim_head(arena_t *arena, arena_chunk_t *chunk,
     arena_run_t *run, size_t oldsize, size_t newsize);
@@ -3594,10 +3594,12 @@ arena_run_alloc(arena_t *arena, arena_bin_t *bin, size_t size, bool large,
 }
 
 static void
-arena_purge(arena_t *arena)
+arena_purge(arena_t *arena, bool all)
 {
 	arena_chunk_t *chunk;
 	size_t i, npages;
+	/* If all is set purge all dirty pages. */
+	size_t dirty_max = all ? 1 : opt_dirty_max;
 #ifdef MALLOC_DEBUG
 	size_t ndirty = 0;
 	rb_foreach_begin(arena_chunk_t, link_dirty, &arena->chunks_dirty,
@@ -3606,7 +3608,7 @@ arena_purge(arena_t *arena)
 	} rb_foreach_end(arena_chunk_t, link_dirty, &arena->chunks_dirty, chunk)
 	assert(ndirty == arena->ndirty);
 #endif
-	RELEASE_ASSERT(arena->ndirty > opt_dirty_max);
+	RELEASE_ASSERT(all || (arena->ndirty > opt_dirty_max));
 
 #ifdef MALLOC_STATS
 	arena->stats.npurge++;
@@ -3618,7 +3620,7 @@ arena_purge(arena_t *arena)
 	 * number of system calls, even if a chunk has only been partially
 	 * purged.
 	 */
-	while (arena->ndirty > (opt_dirty_max >> 1)) {
+	while (arena->ndirty > (dirty_max >> 1)) {
 #ifdef MALLOC_DOUBLE_PURGE
 		bool madvised = false;
 #endif
@@ -3675,7 +3677,7 @@ arena_purge(arena_t *arena)
 				arena->stats.nmadvise++;
 				arena->stats.purged += npages;
 #endif
-				if (arena->ndirty <= (opt_dirty_max >> 1))
+				if (arena->ndirty <= (dirty_max >> 1))
 					break;
 			}
 		}
@@ -3800,7 +3802,7 @@ arena_run_dalloc(arena_t *arena, arena_run_t *run, bool dirty)
 
 	/* Enforce opt_dirty_max. */
 	if (arena->ndirty > opt_dirty_max)
-		arena_purge(arena);
+		arena_purge(arena, false);
 }
 
 static void
@@ -6854,6 +6856,21 @@ _msize(const void *ptr)
 }
 #endif
 
+void
+jemalloc_free_dirty_pages(void)
+{
+	size_t i;
+	for (i = 0; i < narenas; i++) {
+		arena_t *arena = arenas[i];
+
+		if (arena != NULL) {
+			malloc_spin_lock(&arena->lock);
+			arena_purge(arena, true);
+			malloc_spin_unlock(&arena->lock);
+		}
+	}
+}
+
 /*
  * End non-standard functions.
  */
diff --git a/memory/mozjemalloc/jemalloc.h b/memory/mozjemalloc/jemalloc.h
index f91d3e4f3e8b..9af11c8d2ea4 100644
--- a/memory/mozjemalloc/jemalloc.h
+++ b/memory/mozjemalloc/jemalloc.h
@@ -116,6 +116,20 @@ static inline void jemalloc_purge_freed_pages() { }
 void    jemalloc_purge_freed_pages();
 #endif
 
+/*
+ * Free all unused dirty pages in all arenas. Calling this function will slow
+ * down subsequent allocations so it is recommended to use it only when
+ * memory needs to be reclaimed at all costs (see bug 805855). This function
+ * provides functionality similar to mallctl("arenas.purge") in jemalloc 3.
+ */
+
+#if !defined(MOZ_NATIVE_JEMALLOC)
+#if defined(MOZ_MEMORY_LINUX) || defined(MOZ_MEMORY_BSD)
+__attribute__((weak))
+#endif /* defined(MOZ_MEMORY_LINUX) || defined(MOZ_MEMORY_BSD) */
+void    jemalloc_free_dirty_pages();
+#endif /* !defined(MOZ_NATIVE_JEMALLOC) */
+
 #ifdef __cplusplus
 } /* extern "C" */
 #endif
diff --git a/mozglue/build/mozglue.def.in b/mozglue/build/mozglue.def.in
index 6b27f88d2599..6faa4d37bf74 100644
--- a/mozglue/build/mozglue.def.in
+++ b/mozglue/build/mozglue.def.in
@@ -24,6 +24,7 @@ EXPORTS
   je_malloc_good_size
 #endif
   jemalloc_stats
+  jemalloc_free_dirty_pages
   ; A hack to work around the CRT (see giant comment in Makefile.in)
   frex=je_dumb_free_thunk
 #endif