Re: [PATCH -v2] mm: Don't use radix tree writeback tags for pages in swap cache

[Andrew Morton]

On Wed, 31 Aug 2016 10:14:59 +0100 Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:

> >    2506952 __  2%     +28.1%    3212076 __  7%  vm-scalability.throughput
> >    1207402 __  7%     +22.3%    1476578 __  6%  vmstat.swap.so
> >      10.86 __ 12%     -23.4%       8.31 __ 16%  perf-profile.cycles-pp._raw_spin_lock_irq.__add_to_swap_cache.add_to_swap_cache.add_to_swap.shrink_page_list
> >      10.82 __ 13%     -33.1%       7.24 __ 14%  perf-profile.cycles-pp._raw_spin_lock_irqsave.__remove_mapping.shrink_page_list.shrink_inactive_list.shrink_zone_memcg
> >      10.36 __ 11%    -100.0%       0.00 __ -1%  perf-profile.cycles-pp._raw_spin_lock_irqsave.__test_set_page_writeback.bdev_write_page.__swap_writepage.swap_writepage
> >      10.52 __ 12%    -100.0%       0.00 __ -1%  perf-profile.cycles-pp._raw_spin_lock_irqsave.test_clear_page_writeback.end_page_writeback.page_endio.pmem_rw_page
> > 
> 
> I didn't see anything wrong with the patch but it's worth highlighting
> that this hunk means we are now out of GFP bits.

Well ugh.  What are we to do about that?

Sigh.  This?


From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Subject: mm: check that we haven't used more than 32 bits in address_space.flags

After "mm: don't use radix tree writeback tags for pages in swap cache",
all the flags are now used up on 32-bit builds.

Add a build-time assertion to prevent 64-bit developers from accidentally
breaking things.

Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 include/linux/pagemap.h |    2 ++
 init/main.c             |    4 ++++
 2 files changed, 6 insertions(+)

diff -puN include/linux/pagemap.h~mm-check-that-we-havent-used-more-than-32-bits-in-address_spaceflags include/linux/pagemap.h
--- a/include/linux/pagemap.h~mm-check-that-we-havent-used-more-than-32-bits-in-address_spaceflags
+++ a/include/linux/pagemap.h
@@ -27,6 +27,8 @@ enum mapping_flags {
 	AS_EXITING	= __GFP_BITS_SHIFT + 4, /* final truncate in progress */
 	/* writeback related tags are not used */
 	AS_NO_WRITEBACK_TAGS = __GFP_BITS_SHIFT + 5,
+
+	AS_LAST_FLAG,
 };
 
 static inline void mapping_set_error(struct address_space *mapping, int error)
diff -puN init/main.c~mm-check-that-we-havent-used-more-than-32-bits-in-address_spaceflags init/main.c
--- a/init/main.c~mm-check-that-we-havent-used-more-than-32-bits-in-address_spaceflags
+++ a/init/main.c
@@ -59,6 +59,7 @@
 #include <linux/pid_namespace.h>
 #include <linux/device.h>
 #include <linux/kthread.h>
+#include <linux/pagemap.h>
 #include <linux/sched.h>
 #include <linux/signal.h>
 #include <linux/idr.h>
@@ -463,6 +464,9 @@ void __init __weak thread_stack_cache_in
  */
 static void __init mm_init(void)
 {
+	/* Does address_space.flags still fit into a 32-bit ulong? */
+	BUILD_BUG_ON(AS_LAST_FLAG > 32);
+
 	/*
 	 * page_ext requires contiguous pages,
 	 * bigger than MAX_ORDER unless SPARSEMEM.
_