linux-kernel - Re: [PATCH] mm: vmalloc: don't remove inexistent guard hole in remove_vm

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Date:	Thu, 12 Nov 2015 18:55:50 +0300
From:	Andrey Ryabinin <ryabinin.a.a@...il.com>
To:	Jerome Marchand <jmarchan@...hat.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: vmalloc: don't remove inexistent guard hole in remove_vm_area()

2015-11-12 18:17 GMT+03:00 Jerome Marchand <jmarchan@...hat.com>:
> Commit 71394fe50146 ("mm: vmalloc: add flag preventing guard hole
> allocation") missed a spot. Currently remove_vm_area() decreases
> vm->size to remove the guard hole page, even when it isn't present.
> This patch only decreases vm->size when VM_NO_GUARD isn't set.
>
> Signed-off-by: Jerome Marchand <jmarchan@...hat.com>
> ---
>  mm/vmalloc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index d045634..1388c3d 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1443,7 +1443,8 @@ struct vm_struct *remove_vm_area(const void *addr)
>                 vmap_debug_free_range(va->va_start, va->va_end);
>                 kasan_free_shadow(vm);
>                 free_unmap_vmap_area(va);
> -               vm->size -= PAGE_SIZE;
> +               if (!(vm->flags & VM_NO_GUARD))
> +                       vm->size -= PAGE_SIZE;
>

I'd fix this in another way. I think that remove_vm_area() shouldn't
change vm's size, IMO it doesn't make sense.
The only caller who cares about vm's size after removing is __vunmap():
         area = remove_vm_area(addr);
         ....
         debug_check_no_locks_freed(addr, area->size);
         debug_check_no_obj_freed(addr, area->size);

We already have proper get_vm_area_size() helper which takes
VM_NO_GUARD into account.
So I think we should use that helper for debug_check_no_*() and just
remove 'vm->size -= PAGE_SIZE;' line
from remove_vm_area()



>                 return vm;
>         }
> --
> 2.4.3
>
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