Re: [PATCH 2/5] mm/hugetlb: Remove prepare_hugepage_range()

Peter Xu
2025-06-13T16:24:51Z

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@kernel.org>
> > Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
> > Cc: Muchun Song <muchun.song@linux.dev>
> > Cc: Oscar Salvador <osalvador@suse.de>
> > Cc: loongarch@lists.linux.dev
> > Cc: linux-mips@vger.kernel.org
> > Signed-off-by: Peter Xu <peterx@redhat.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