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Subject: Re: [PATCH] vfio/type1: remove hugepage checks in
 is_invalid_reserved_pfn()
To:     alex.williamson@redhat.com
Cc:     aarcange@redhat.com, linux-kernel@vger.kernel.org
References: <1d6b7e1c40783f2db4c6cb15bf679a94222ec6a3.1570073993.git.luoben@linux.alibaba.com>
 <20191003164133.GG13922@redhat.com>
From:   Ben Luo <luoben@linux.alibaba.com>
Message-ID: <ae787a52-a7ed-f7b8-074f-ab8883037ac0@linux.alibaba.com>
Date:   Fri, 18 Oct 2019 14:42:32 +0800
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A friendly reminder :)


Thanks,

     Ben

在 2019/10/4 上午12:41, Andrea Arcangeli 写道:
> On Thu, Oct 03, 2019 at 11:49:42AM +0800, Ben Luo wrote:
>> Currently, no hugepage split code can transfer the reserved bit
>> from head to tail during the split, so checking the head can't make
>> a difference in a racing condition with hugepage spliting.
>>
>> The buddy wouldn't allow a driver to allocate an hugepage if any
>> subpage is reserved in the e820 map at boot, if any driver sets the
>> reserved bit of head page before mapping the hugepage in userland,
>> it needs to set the reserved bit in all subpages to be safe.
>>
>> Signed-off-by: Ben Luo <luoben@linux.alibaba.com>
> Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
>
>
>> ---
>>   drivers/vfio/vfio_iommu_type1.c | 26 ++++----------------------
>>   1 file changed, 4 insertions(+), 22 deletions(-)
>>
>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>> index 054391f..e2019ba 100644
>> --- a/drivers/vfio/vfio_iommu_type1.c
>> +++ b/drivers/vfio/vfio_iommu_type1.c
>> @@ -287,31 +287,13 @@ static int vfio_lock_acct(struct vfio_dma *dma, long npage, bool async)
>>    * Some mappings aren't backed by a struct page, for example an mmap'd
>>    * MMIO range for our own or another device.  These use a different
>>    * pfn conversion and shouldn't be tracked as locked pages.
>> + * For compound pages, any driver that sets the reserved bit in head
>> + * page needs to set the reserved bit in all subpages to be safe.
>>    */
>>   static bool is_invalid_reserved_pfn(unsigned long pfn)
>>   {
>> -	if (pfn_valid(pfn)) {
>> -		bool reserved;
>> -		struct page *tail = pfn_to_page(pfn);
>> -		struct page *head = compound_head(tail);
>> -		reserved = !!(PageReserved(head));
>> -		if (head != tail) {
>> -			/*
>> -			 * "head" is not a dangling pointer
>> -			 * (compound_head takes care of that)
>> -			 * but the hugepage may have been split
>> -			 * from under us (and we may not hold a
>> -			 * reference count on the head page so it can
>> -			 * be reused before we run PageReferenced), so
>> -			 * we've to check PageTail before returning
>> -			 * what we just read.
>> -			 */
>> -			smp_rmb();
>> -			if (PageTail(tail))
>> -				return reserved;
>> -		}
>> -		return PageReserved(tail);
>> -	}
>> +	if (pfn_valid(pfn))
>> +		return PageReserved(pfn_to_page(pfn));
>>   
>>   	return true;
>>   }
>> -- 
>> 1.8.3.1
>>