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SubjectRe: [PATCH][mm/memory.c]: transparent hugepage check condition missed


>>> On 11/1/2011 at 09:18 AM, in message <1320110288.22361.190.camel@sli10-conroe>,
Shaohua Li <shaohua.li@intel.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@suse.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, then THP will be confused.

There is already a defination:

#define VM_HUGEPAGE 0x01000000 /* MADV_HUGEPAGE marked this vma */

maybe,this can be the flag to identity a hugepage.But the comment marked it only stands for MADV_HUGEPAGE,
and it's still a hugepage.So, I suggest to add the check condition !vma->vm_ops, or turn to
use VM_HUGEPAGE as the flag.
or
adjust the logic to:
(transparent_hugepage_enabled() use the VM_HUGEPAGE flag)

if(transparent_hugepage_enabled(vma)){
if (pmd_none(*pmd){
...
}
else
{
...
}
}


the original logic is:

if (pmd_none(*pmd) && transparent_hugepage_enabled(vma))
{
...
}
else
{
...
}



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