[PATCH] mm: thp: default defrag mode to defer+madvise

[Ferran Duarri <ferran.duarri@me.com>]

transparent_hugepage_flags unconditionally sets
TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG, so the built-in defrag mode
is always "madvise" regardless of which of CONFIG_TRANSPARENT_HUGEPAGE_
ALWAYS or _MADVISE was selected. Those Kconfig options choose whether
THP is applied by default; they do not express a preference about how
hard the allocator should work to produce a huge page, yet the defrag
default is pinned as if they did.

In "madvise" mode a fault on a MADV_HUGEPAGE region performs direct
compaction in the fault path. Paired with transparent_hugepage=madvise
that is a bounded cost, since only regions that asked for it are
affected. Paired with transparent_hugepage=always every anonymous fault
becomes eligible, and under memory pressure the faulting thread can
stall in compaction.

"defer+madvise" keeps the same set of regions eligible for huge pages
and the same allocation attempt, but on failure it wakes kswapd and
khugepaged to compact in the background instead of doing it inline. The
fault proceeds with small pages and the region is collapsed later. This
is the configuration long recommended to users running THP=always with
large anonymous working sets, and it is a strictly weaker stall
guarantee to make the default.

This changes only the compiled-in default;
/sys/kernel/mm/transparent_hugepage/defrag continues to accept every
existing mode, including the current "madvise" behaviour.

No stall measurement is offered with this patch. On the machine that
prompted it the fault path never reaches direct compaction at all:
thp_fault_alloc is 60682 against thp_fault_fallback 0, and compact_stall
is 0, because memory has stayed abundant enough that no huge-page
allocation has had to fall back. That makes the box unable to testify
either way, and a number gathered under those conditions would measure
nothing. The argument above is a correctness one about which knob the
Kconfig choice is entitled to set, and it should be judged on that.

Signed-off-by: Ferran Duarri <ferran.duarri@me.com>
---
 mm/huge_memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index b118bcd392cb..749f669cca56 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -63,7 +63,7 @@ unsigned long transparent_hugepage_flags __read_mostly =
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE_MADVISE
 	(1<<TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG)|
 #endif
-	(1<<TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG)|
+	(1<<TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_OR_MADV_FLAG)|
 	(1<<TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG)|
 	(1<<TRANSPARENT_HUGEPAGE_USE_ZERO_PAGE_FLAG);