shmem: replace page if mapping excludes its zone

The GMA500 GPU driver uses GEM shmem objects, but with a new twist: the
backing RAM has to be below 4GB.  Not a problem while the boards
supported only 4GB: but now Intel's D2700MUD boards support 8GB, and
their GMA3600 is managed by the GMA500 driver.

shmem/tmpfs has never pretended to support hardware restrictions on the
backing memory, but it might have appeared to do so before v3.1, and
even now it works fine until a page is swapped out then back in.  When
read_cache_page_gfp() supplied a freshly allocated page for copy, that
compensated for whatever choice might have been made by earlier swapin
readahead; but swapoff was likely to destroy the illusion.

We'd like to continue to support GMA500, so now add a new
shmem_should_replace_page() check on the zone when about to move a page
from swapcache to filecache (in swapin and swapoff cases), with
shmem_replace_page() to allocate and substitute a suitable page (given
gma500/gem.c's mapping_set_gfp_mask GFP_KERNEL | __GFP_DMA32).

This does involve a minor extension to mem_cgroup_replace_page_cache()
(the page may or may not have already been charged); and I've removed a
comment and call to mem_cgroup_uncharge_cache_page(), which in fact is
always a no-op while PageSwapCache.

Also removed optimization of an unlikely path in shmem_getpage_gfp(),
now that we need to check PageSwapCache more carefully (a racing caller
might already have made the copy).  And at one point shmem_unuse_inode()
needs to use the hitherto private page_swapcount(), to guard against
racing with inode eviction.

It would make sense to extend shmem_should_replace_page(), to cover
cpuset and NUMA mempolicy restrictions too, but set that aside for now:
needs a cleanup of shmem mempolicy handling, and more testing, and ought
to handle swap faults in do_swap_page() as well as shmem.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Christoph Hellwig <hch@infradead.org>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Stephane Marchesin <marcheu@chromium.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Rob Clark <rob.clark@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 4 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 4f71219cc53..d7ce417cae7 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3373,7 +3373,7 @@ void mem_cgroup_end_migration(struct mem_cgroup *memcg,
 void mem_cgroup_replace_page_cache(struct page *oldpage,
 				  struct page *newpage)
 {
-	struct mem_cgroup *memcg;
+	struct mem_cgroup *memcg = NULL;
 	struct page_cgroup *pc;
 	enum charge_type type = MEM_CGROUP_CHARGE_TYPE_CACHE;
 
@@ -3383,11 +3383,20 @@ void mem_cgroup_replace_page_cache(struct page *oldpage,
 	pc = lookup_page_cgroup(oldpage);
 	/* fix accounting on old pages */
 	lock_page_cgroup(pc);
-	memcg = pc->mem_cgroup;
-	mem_cgroup_charge_statistics(memcg, false, -1);
-	ClearPageCgroupUsed(pc);
+	if (PageCgroupUsed(pc)) {
+		memcg = pc->mem_cgroup;
+		mem_cgroup_charge_statistics(memcg, false, -1);
+		ClearPageCgroupUsed(pc);
+	}
 	unlock_page_cgroup(pc);
 
+	/*
+	 * When called from shmem_replace_page(), in some cases the
+	 * oldpage has already been charged, and in some cases not.
+	 */
+	if (!memcg)
+		return;
+
 	if (PageSwapBacked(oldpage))
 		type = MEM_CGROUP_CHARGE_TYPE_SHMEM;