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

[Ferran]

Bad computer. Go sit in the corner, and think about your life!

https://youtu.be/_8c58JZQ4Bs?si=mA66nH1swfK9eRML


have a nice weekend,

Ferran


On 8/21/26 6:53 PM, Lorenzo Stoakes (ARM) wrote:
> No. We don't randomly change core mm defaults like this especially not on the
> behest of AI schlop...
>
> On Thu, Aug 20, 2026 at 09:08:23PM +0200, Ferran Duarri wrote:
> > 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.
> It's not pinned. Distros change these as they please.
>
> That's where any such change should be.
>
> > 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
> Not really that bounded in practice.
>
> > affected. Paired with transparent_hugepage=always every anonymous fault
> > becomes eligible, and under memory pressure the faulting thread can
> > stall in compaction.
> What? No?
>
> > "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
> If we felt it was right to set this a default we would have...
>
> > 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.
> None of this judstifies anything.
>
> > 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.
> This sentence is truly ludicrious...!
>
> Made me laugh a bit though which is something at least.
>
> > Signed-off-by: Ferran Duarri <ferran.duarri@xxxxxx>
> You:
>
> - Only started posting patches yesterday
> - Have posted patches all over the kernel
> - Post walls of text that read exactly like they are AI-generated
> - Are trying to change a fundamental default, not as an RFC, not citing any
>    actual workloads
>
> Go away and read:
>
> https://docs.kernel.org/process/coding-assistants.html
> https://docs.kernel.org/process/generated-content.html
>
> Then ideally just go away.
>
> We don't want AI slop patches from people who have no understanding of what
> they're changing, thanks!
>
> > ---
> >   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);
> --
> Cheers, Lorenzo