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

Liam R. Howlett
2025-06-14T04:11:53Z

* Peter Xu <peterx@redhat.com> [691231 23:00]:
> 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.

I'm not as confident..

>
> 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;

arch/mips/include/asm/processor.h:#define STACK_TOP mips_stack_top()

unsigned long mips_stack_top(void)
{
unsigned long top = TASK_SIZE & PAGE_MASK;

if (IS_ENABLED(CONFIG_MIPS_FP_SUPPORT)) {
/* One page for branch delay slot "emulation" */
top -= PAGE_SIZE;
}

/* Space for the VDSO, data page & GIC user page */
top -= PAGE_ALIGN(current->thread.abi->vdso->size);
top -= PAGE_SIZE;
top -= mips_gic_present() ? PAGE_SIZE : 0;

/* Space for cache colour alignment */
if (cpu_has_dc_aliases)
top -= shm_align_mask + 1;

/* Space to randomize the VDSO base */
if (current->flags & PF_RANDOMIZE)
top -= VDSO_RANDOMIZE_SIZE;

return top;
}

This seems different than TASK_SIZE.

Code is from:
commit ea7e0480a4b695d0aa6b3fa99bd658a003122113
Author: Paul Burton <paulburton@kernel.org>
Date: Tue Sep 25 15:51:26 2018 -0700

> - if (len > task_size)
> - return -ENOMEM;
> - if (task_size - len < addr)
> - return -EINVAL;
> - return 0;
> -}
> -

Unfortunately, the commit message for the addition of this code are not
helpful.

commit 50a41ff292fafe1e937102be23464b54fed8b78c
Author: David Daney <ddaney@caviumnetworks.com>
Date: Wed May 27 17:47:42 2009 -0700

... But the dates are helpful. This code used to use:
#define STACK_TOP ((TASK_SIZE & PAGE_MASK) - PAGE_SIZE)

It's not exactly task size either.

I don't think this is an issue to remove this check because the overflow
should be caught later (or trigger the opposite search). But it's not
clear why STACK_TOP was done in the first place.. Maybe just because we
know the overflow here would be an issue later, but then we'd avoid the
opposite search - and maybe that's the point?

Either way, your comment about the same check existing doesn't seem
correct.

I haven't checked loong arch, but I'd be willing to wager this was just
cloned mips code... because this happens so much.

...

Thanks,
Liam