linux-kernel - Re: [PATCH] mm: revert mremap pud

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Message-ID: <525D2B15.8060503@asianux.com>
Date:	Tue, 15 Oct 2013 19:46:29 +0800
From:	Chen Gang <gang.chen@...anux.com>
To:	Hugh Dickins <hughd@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: revert mremap pud_free anti-fix

On 10/15/2013 06:34 PM, Hugh Dickins wrote:
> Revert 1ecfd533f4c5 ("mm/mremap.c: call pud_free() after fail calling
> pmd_alloc()").  The original code was correct: pud_alloc(), pmd_alloc(),
> pte_alloc_map() ensure that the pud, pmd, pt is already allocated, and
> seldom do they need to allocate; on failure, upper levels are freed if
> appropriate by the subsequent do_munmap().  Whereas 1ecfd533f4c5 did an
> unconditional pud_free() of a most-likely still-in-use pud: saved only
> by the near-impossiblity of pmd_alloc() failing.
> 

What you said above sounds reasonable to me,  but better to provide the
information below:

 - pud_free() for pgd_alloc() in "arch/arm/mm/pgd.c".

 - pud_free() for init_stub_pte() in "arch/um/kernel/skas/mmu.c".

 - more details about do_munmap(), (e.g. do it need mm->page_table_lock)
   or more details about the demo "most-likely still-in-use pud ...".


Hmm... I am not quite sure about the 3 things, and I will/should
continue analysing/learning about them, but better to get your reply. :-)

Thanks.

> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> ---
> 
>  mm/mremap.c |    5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> --- 3.12-rc5/mm/mremap.c	2013-09-16 17:37:56.841072270 -0700
> +++ linux/mm/mremap.c	2013-10-15 03:07:09.140091599 -0700
> @@ -25,7 +25,6 @@
>  #include <asm/uaccess.h>
>  #include <asm/cacheflush.h>
>  #include <asm/tlbflush.h>
> -#include <asm/pgalloc.h>
>  
>  #include "internal.h"
>  
> @@ -63,10 +62,8 @@ static pmd_t *alloc_new_pmd(struct mm_st
>  		return NULL;
>  
>  	pmd = pmd_alloc(mm, pud, addr);
> -	if (!pmd) {
> -		pud_free(mm, pud);
> +	if (!pmd)
>  		return NULL;
> -	}
>  
>  	VM_BUG_ON(pmd_trans_huge(*pmd));
>  
> 
> 


-- 
Chen Gang
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