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

Peter Xu
2025-06-17T21:07:40Z

On Sat, Jun 14, 2025 at 12:11:22AM -0400, Liam R. Howlett wrote:
> * 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 will fix up the commit message to mention both archs:

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, even though it may not
be 100% identical checks.

For example, for both of the architectures, there will be a trivial
difference on how stack top was defined. The old code uses STACK_TOP which
may be slightly smaller than TASK_SIZE on either of them, but the hope is
that shouldn't be a problem.

It means the whole API is pretty much obsolete at least now, remove it
completely.

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

They define STACK_TOP differently, but AFAIU there're some duplications in
pattern of the two archs.

Please let me know if the fixed commit message works for you above, thanks.

--
Peter Xu