linux-kernel - Re: [PATCH 2/5] mm/hugetlb: Remove prepare_hugepage

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Message-ID: <aExQy6xMDc9Igm5v@x1.local>
Date: Fri, 13 Jun 2025 12:24:43 -0400
From: Peter Xu <peterx@...hat.com>
To: Zi Yan <ziy@...dia.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, kvm@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alex Williamson <alex.williamson@...hat.com>,
	Jason Gunthorpe <jgg@...dia.com>, Alex Mastro <amastro@...com>,
	David Hildenbrand <david@...hat.com>,
	Nico Pache <npache@...hat.com>, Huacai Chen <chenhuacai@...nel.org>,
	Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
	Muchun Song <muchun.song@...ux.dev>,
	Oscar Salvador <osalvador@...e.de>, loongarch@...ts.linux.dev,
	linux-mips@...r.kernel.org
Subject: Re: [PATCH 2/5] mm/hugetlb: Remove prepare_hugepage_range()

On Fri, Jun 13, 2025 at 11:13:50AM -0400, Zi Yan wrote:
> On 13 Jun 2025, at 9:41, Peter Xu wrote:
> 
> > Only mips and loongarch implemented this API, however what it does was
> > checking against stack overflow for either len or addr.  That's already
> > done in arch's arch_get_unmapped_area*() functions, hence not needed.
> >
> > It means the whole API is pretty much obsolete at least now, remove it
> > completely.
> >
> > Cc: Huacai Chen <chenhuacai@...nel.org>
> > Cc: Thomas Bogendoerfer <tsbogend@...ha.franken.de>
> > Cc: Muchun Song <muchun.song@...ux.dev>
> > Cc: Oscar Salvador <osalvador@...e.de>
> > Cc: loongarch@...ts.linux.dev
> > Cc: linux-mips@...r.kernel.org
> > Signed-off-by: Peter Xu <peterx@...hat.com>
> > ---
> >  arch/loongarch/include/asm/hugetlb.h | 14 --------------
> >  arch/mips/include/asm/hugetlb.h      | 14 --------------
> >  fs/hugetlbfs/inode.c                 |  8 ++------
> >  include/asm-generic/hugetlb.h        |  8 --------
> >  include/linux/hugetlb.h              |  6 ------
> >  5 files changed, 2 insertions(+), 48 deletions(-)
> >
> > diff --git a/arch/loongarch/include/asm/hugetlb.h b/arch/loongarch/include/asm/hugetlb.h
> > index 4dc4b3e04225..ab68b594f889 100644
> > --- a/arch/loongarch/include/asm/hugetlb.h
> > +++ b/arch/loongarch/include/asm/hugetlb.h
> > @@ -10,20 +10,6 @@
> >
> >  uint64_t pmd_to_entrylo(unsigned long pmd_val);
> >
> > -#define __HAVE_ARCH_PREPARE_HUGEPAGE_RANGE
> > -static inline int prepare_hugepage_range(struct file *file,
> > -					 unsigned long addr,
> > -					 unsigned long len)
> > -{
> > -	unsigned long task_size = STACK_TOP;
> > -
> > -	if (len > task_size)
> > -		return -ENOMEM;
> > -	if (task_size - len < addr)
> > -		return -EINVAL;
> > -	return 0;
> > -}
> > -
> >  #define __HAVE_ARCH_HUGE_PTE_CLEAR
> >  static inline void huge_pte_clear(struct mm_struct *mm, unsigned long addr,
> >  				  pte_t *ptep, unsigned long sz)
> > diff --git a/arch/mips/include/asm/hugetlb.h b/arch/mips/include/asm/hugetlb.h
> > index fbc71ddcf0f6..8c460ce01ffe 100644
> > --- a/arch/mips/include/asm/hugetlb.h
> > +++ b/arch/mips/include/asm/hugetlb.h
> > @@ -11,20 +11,6 @@
> >
> >  #include <asm/page.h>
> >
> > -#define __HAVE_ARCH_PREPARE_HUGEPAGE_RANGE
> > -static inline int prepare_hugepage_range(struct file *file,
> > -					 unsigned long addr,
> > -					 unsigned long len)
> > -{
> > -	unsigned long task_size = STACK_TOP;
> > -
> > -	if (len > task_size)
> > -		return -ENOMEM;
> 
> arch_get_unmapped_area_topdown() has this check.
> 
> > -	if (task_size - len < addr)
> > -		return -EINVAL;
> 
> For this one, arch_get_unmapped_area_topdown() instead will try to
> provide a different addr if the check fails.
> 
> So this patch changes the original code behavior, right?

It almost shouldn't change.  Note that prepare_hugepage_range() is only
used for MAP_FIXED before this patch:

hugetlb_get_unmapped_area():
        if (flags & MAP_FIXED) {
                if (addr & ~huge_page_mask(h))
                        return -EINVAL;
                if (prepare_hugepage_range(file, addr, len))
                        return -EINVAL;
        }

Then for MAP_FIXED, on MIPS:

arch_get_unmapped_area_common():
        ...
	if (flags & MAP_FIXED) {
		/* Even MAP_FIXED mappings must reside within TASK_SIZE */
		if (TASK_SIZE - len < addr)
			return -EINVAL;
                ...
        }

But if we want to be super accurate, it's indeed different, in that the old
hugetlb code was checking stack top with STACK_TOP, which is
mips_stack_top() for MIPS: it's a value that might be slightly less than
TASK_SIZE..

So strictly speaking, there's indeed a trivial difference on the oddity of
defining stack top, but my guess is nothing will be affected.  I can add
some explanation into the commit message in that case.

Thanks,

-- 
Peter Xu