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

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* 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