linux-kernel - Re: [PATCH v1 06/36] mm/page_alloc: reject unreasonable folio/compound page sizes in alloc_contig_range

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Message-ID: <34edaa0d-0d5f-4041-9a3d-fb5b2dd584e8@lucifer.local>
Date: Fri, 29 Aug 2025 13:31:41 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, Zi Yan <ziy@...dia.com>,
        SeongJae Park <sj@...nel.org>, Alexander Potapenko <glider@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
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Subject: Re: [PATCH v1 06/36] mm/page_alloc: reject unreasonable
 folio/compound page sizes in alloc_contig_range_noprof()

On Fri, Aug 29, 2025 at 12:06:21PM +0200, David Hildenbrand wrote:
> On 28.08.25 16:37, Lorenzo Stoakes wrote:
> > On Thu, Aug 28, 2025 at 12:01:10AM +0200, David Hildenbrand wrote:
> > > Let's reject them early, which in turn makes folio_alloc_gigantic() reject
> > > them properly.
> > >
> > > To avoid converting from order to nr_pages, let's just add MAX_FOLIO_ORDER
> > > and calculate MAX_FOLIO_NR_PAGES based on that.
> > >
> > > Reviewed-by: Zi Yan <ziy@...dia.com>
> > > Acked-by: SeongJae Park <sj@...nel.org>
> > > Signed-off-by: David Hildenbrand <david@...hat.com>
> >
> > Some nits, but overall LGTM so:
> >
> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> >
> > > ---
> > >   include/linux/mm.h | 6 ++++--
> > >   mm/page_alloc.c    | 5 ++++-
> > >   2 files changed, 8 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > index 00c8a54127d37..77737cbf2216a 100644
> > > --- a/include/linux/mm.h
> > > +++ b/include/linux/mm.h
> > > @@ -2055,11 +2055,13 @@ static inline long folio_nr_pages(const struct folio *folio)
> > >
> > >   /* Only hugetlbfs can allocate folios larger than MAX_ORDER */
> > >   #ifdef CONFIG_ARCH_HAS_GIGANTIC_PAGE
> > > -#define MAX_FOLIO_NR_PAGES	(1UL << PUD_ORDER)
> > > +#define MAX_FOLIO_ORDER		PUD_ORDER
> > >   #else
> > > -#define MAX_FOLIO_NR_PAGES	MAX_ORDER_NR_PAGES
> > > +#define MAX_FOLIO_ORDER		MAX_PAGE_ORDER
> > >   #endif
> > >
> > > +#define MAX_FOLIO_NR_PAGES	(1UL << MAX_FOLIO_ORDER)
> >
> > BIT()?
>
> I don't think we want to use BIT whenever we convert from order -> folio --
> which is why we also don't do that in other code.

It seems a bit arbitrary, like we open-code this (at risk of making a mistake)
in some places but not others.

>
> BIT() is nice in the context of flags and bitmaps, but not really in the
> context of converting orders to pages.

It's nice for setting a specific bit :)

>
> One could argue that maybe one would want a order_to_pages() helper (that
> could use BIT() internally), but I am certainly not someone that would
> suggest that at this point ...  :)

I mean maybe.

Anyway as I said none of this is massively important, the open-coding here is
correct, just seems silly.

>
> >
> > > +
> > >   /*
> > >    * compound_nr() returns the number of pages in this potentially compound
> > >    * page.  compound_nr() can be called on a tail page, and is defined to
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index baead29b3e67b..426bc404b80cc 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -6833,6 +6833,7 @@ static int __alloc_contig_verify_gfp_mask(gfp_t gfp_mask, gfp_t *gfp_cc_mask)
> > >   int alloc_contig_range_noprof(unsigned long start, unsigned long end,
> > >   			      acr_flags_t alloc_flags, gfp_t gfp_mask)

Funny btw th

> > >   {
> > > +	const unsigned int order = ilog2(end - start);
> > >   	unsigned long outer_start, outer_end;
> > >   	int ret = 0;
> > >
> > > @@ -6850,6 +6851,9 @@ int alloc_contig_range_noprof(unsigned long start, unsigned long end,
> > >   					    PB_ISOLATE_MODE_CMA_ALLOC :
> > >   					    PB_ISOLATE_MODE_OTHER;
> > >
> > > +	if (WARN_ON_ONCE((gfp_mask & __GFP_COMP) && order > MAX_FOLIO_ORDER))
> > > +		return -EINVAL;
> >
> > Possibly not worth it for a one off, but be nice to have this as a helper function, like:
> >
> > static bool is_valid_order(gfp_t gfp_mask, unsigned int order)
> > {
> > 	return !(gfp_mask & __GFP_COMP) || order <= MAX_FOLIO_ORDER;
> > }
> >
> > Then makes this:
> >
> > 	if (WARN_ON_ONCE(!is_valid_order(gfp_mask, order)))
> > 		return -EINVAL;
> >
> > Kinda self-documenting!
>
> I don't like it -- especially forwarding __GFP_COMP.
>
> is_valid_folio_order() to wrap the order check? Also not sure.

OK, it's not a big deal.

Can we have a comment explaining this though? As people might be confused
as to why we check this here and not elsewhere.
>
> So I'll leave it as is I think.

Right fine.

>
> Thanks for all the review!
>
> --
> Cheers
>
> David / dhildenb
>