linux-kernel - Re: [PATCH 2/2] mm/vmalloc: Add attempt_larger_order

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Message-ID: <aUKb1bL7CUcCWi8V@milan>
Date: Wed, 17 Dec 2025 13:02:29 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: "Uladzislau Rezki (Sony)" <urezki@...il.com>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vishal Moola <vishal.moola@...il.com>, Dev Jain <dev.jain@....com>,
	Baoquan He <bhe@...hat.com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm/vmalloc: Add attempt_larger_order_alloc parameter

> On 16/12/2025 21:19, Uladzislau Rezki (Sony) wrote:
> > Introduce a module parameter to enable or disable the large-order
> > allocation path in vmalloc. High-order allocations are disabled by
> > default so far, but users may explicitly enable them at runtime if
> > desired.
> > 
> > High-order pages allocated for vmalloc are immediately split into
> > order-0 pages and later freed as order-0, which means they do not
> > feed the per-CPU page caches. As a result, high-order attempts tend
> > to bypass the PCP fastpath and fall back to the buddy allocator that
> > can affect performance.
> > 
> > However, when the PCP caches are empty, high-order allocations may
> > show better performance characteristics especially for larger
> > allocation requests.
> 
> I wonder if a better solution would be "allocate order-0 if available in pcp,
> else try large order, else fallback to order-0" Could that provide the best of
> all worlds without needing a configuration knob?
> 
I am not sure, to me it looks like a bit odd. Ideally it would be
good just free it as high-order page and not order-0 peaces.

> > 
> > Since the best strategy is workload-dependent, this patch adds a
> > parameter letting users to choose whether vmalloc should try
> > high-order allocations or stay strictly on the order-0 fastpath.
> > 
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> > ---
> >  mm/vmalloc.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index d3a4725e15ca..f66543896b16 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -43,6 +43,7 @@
> >  #include <asm/tlbflush.h>
> >  #include <asm/shmparam.h>
> >  #include <linux/page_owner.h>
> > +#include <linux/moduleparam.h>
> >  
> >  #define CREATE_TRACE_POINTS
> >  #include <trace/events/vmalloc.h>
> > @@ -3671,6 +3672,9 @@ vm_area_alloc_pages_large_order(gfp_t gfp, int nid, unsigned int order,
> >  	return nr_allocated;
> >  }
> >  
> > +static int attempt_larger_order_alloc;
> > +module_param(attempt_larger_order_alloc, int, 0644);
> 
> Would this be better as a bool? Docs say that you can then specify 0/1, y/n or
> Y/N as the value; that's probably more intuitive?
> 
> nit: I'd favour a shorter name. Perhaps large_order_alloc?
> 
Thanks! We can switch to bool and use shorter name for sure.

--
Uladzislau Rezki