linux-kernel - Re: [PATCH][mm/memory.c]: transparent hugepage check condition missed

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Date:	Tue, 01 Nov 2011 03:18:19 -0600
From:	"Guan Jun He" <gjhe@...e.com>
To:	"Shaohua Li" <shaohua.li@...el.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][mm/memory.c]: transparent hugepage check condition
 missed



>>> On 11/1/2011 at 04:42 PM, in message
<CANejiEVk41X-P+UyMf96jmPrJJ5-_vbubYtnQgaWXY2FLb41iw@...l.gmail.com>, Shaohua
Li <shaohua.li@...el.com> wrote: 
> 2011/11/1 Guan Jun He <gjhe@...e.com>:
>>
>>
>>>>> On 11/1/2011 at 09:18 AM, in message <1320110288.22361.190.camel@...10-conroe>,
>> Shaohua Li <shaohua.li@...el.com> wrote:
>>> On Mon, 2011-10-31 at 16:23 +0800, Guanjun He wrote:
>>>> For the transparent hugepage module still does not support
>>>> tmpfs and cache,the check condition should always be checked
>>>> to make sure that it only affect the anonymous maps, the
>>>> original check condition missed this, this patch is to fix this.
>>>> Otherwise,the hugepage may affect the file-backed maps,
>>>> then the cache for the small-size pages will be unuseful,
>>>> and till now there is still no implementation for hugepage's cache.
>>>>
>>>> Signed-off-by: Guanjun He <gjhe@...e.com>
>>>> ---
>>>>  mm/memory.c |    3 ++-
>>>>  1 files changed, 2 insertions(+), 1 deletions(-)
>>>>
>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>> index a56e3ba..79b85fe 100644
>>>> --- a/mm/memory.c
>>>> +++ b/mm/memory.c
>>>> @@ -3475,7 +3475,8 @@ int handle_mm_fault(struct mm_struct *mm, struct
>>> vm_area_struct *vma,
>>>>              if (pmd_trans_huge(orig_pmd)) {
>>>>                      if (flags & FAULT_FLAG_WRITE &&
>>>>                          !pmd_write(orig_pmd) &&
>>>> -                        !pmd_trans_splitting(orig_pmd))
>>>> +                        !pmd_trans_splitting(orig_pmd) &&
>>>> +                        !vma->vm_ops)
>>>>                              return do_huge_pmd_wp_page(mm, vma, address,
>>>>                                                         pmd, orig_pmd);
>>>>                      return 0;
>>> so if vma->vm_ops != NULL, how could the pmd_trans_huge(orig_pmd) be
>>> true? We never enable THP if vma->vm_ops != NULL.
>> acturally, pmd_trans_huge(orig_pmd) only checks the _PAGE_PSE bits,
>> it's only a pagesize, not a flag to identity a hugepage.
>> If I change my default pagesize to PAGE_PSE,
> Not sure what pagesize means here, assume pmd entry bits.
yes, it's  pmd entry bits.
> how could you make the default 'pagesize' to PAGE_PSE?
That requires some work and not so easy and need hardware support... So, recently it won't come.
But one can easily create the same pmd entry bits for some special use;
as comment above, it's a pmd entry bits, only mark a size, not a flag;
and adjust the logic to use the flag can perfect avoid this potential issuse,
and basically no impact to the current code.




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