[PATCH] [RFC] init/main: fix broken buffer_init when DEFERRED_STRUCT_PAGE_INIT set

[Lin Feng]

In the booting phase if CONFIG_DEFERRED_STRUCT_PAGE_INIT is set,
we have following callchain:

start_kernel
...
  mm_init
    mem_init
     memblock_free_all
       reset_all_zones_managed_pages
       free_low_memory_core_early
...
  buffer_init
    nr_free_buffer_pages
      zone->managed_pages
...
  rest_init
    kernel_init
      kernel_init_freeable
        page_alloc_init_late
          kthread_run(deferred_init_memmap, NODE_DATA(nid), "pgdatinit%d", nid);
          wait_for_completion(&pgdat_init_all_done_comp);
          ...
          files_maxfiles_init

It's clear that buffer_init depends on zone->managed_pages, but it's reset
in reset_all_zones_managed_pages after that pages are readded into
 zone->managed_pages, but when buffer_init runs this process is half done
 and most of them will finally be added till deferred_init_memmap done.
In large memory couting of nr_free_buffer_pages drifts too much, also
drifting from kernels to kernels on same hardware.

Fix is simple, it delays buffer_init run till deferred_init_memmap all done.

But as corrected by this patch, max_buffer_heads becomes very large,
the value is roughly as many as 4 times of totalram_pages, formula:
max_buffer_heads = nrpages * (10%) * (PAGE_SIZE / sizeof(struct buffer_head));

Say in a 64GB memory box we have 16777216 pages, then max_buffer_heads
turns out to be roughly 67,108,864.
In common cases, should a buffer_head be mapped to one page/block(4KB)?
So max_buffer_heads never exceeds totalram_pages.
IMO it's likely to make buffer_heads_over_limit bool value alwasy false,
then make codes 'if (buffer_heads_over_limit)' test in vmscan unnecessary.
Correct me if it's not true.

So this patch will change the original behavior related to
buffer_heads_over_limit in vmscan since we used a half done value
of zone->managed_pages before, or should we use a smaller factor(<10%) in
previous formula.

Signed-off-by: Lin Feng <linf@xxxxxxxxxx>
---
 init/main.c     | 2 --
 mm/page_alloc.c | 3 +++
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/init/main.c b/init/main.c
index 20baced721ad..a3f7c3416286 100644
--- a/init/main.c
+++ b/init/main.c
@@ -58,7 +58,6 @@
 #include <linux/rmap.h>
 #include <linux/mempolicy.h>
 #include <linux/key.h>
-#include <linux/buffer_head.h>
 #include <linux/page_ext.h>
 #include <linux/debug_locks.h>
 #include <linux/debugobjects.h>
@@ -1034,7 +1033,6 @@ asmlinkage __visible void __init __no_sanitize_address start_kernel(void)
 	fork_init();
 	proc_caches_init();
 	uts_ns_init();
-	buffer_init();
 	key_init();
 	security_init();
 	dbg_late_init();
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index eaa227a479e4..2931d706fb52 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -70,6 +70,7 @@
 #include <linux/psi.h>
 #include <linux/padata.h>
 #include <linux/khugepaged.h>
+#include <linux/buffer_head.h>
 
 #include <asm/sections.h>
 #include <asm/tlbflush.h>
@@ -2103,6 +2104,8 @@ void __init page_alloc_init_late(void)
 	files_maxfiles_init();
 #endif
 
+	buffer_init();
+
 	/* Discard memblock private memory */
 	memblock_discard();
 
-- 
2.25.4