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 --- 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<