Re: [RFC PATCH 2/4] iommu/vt-d: Add first level page table interfaces

From: Lu Baolu <baolu.lu@linux.intel.com>
To: Peter Xu <peterx@redhat.com>
Cc: kevin.tian@intel.com, Yi Sun <yi.y.sun@linux.intel.com>,
	ashok.raj@intel.com, kvm@vger.kernel.org,
	sanjay.k.kumar@intel.com, iommu@lists.linux-foundation.org,
	linux-kernel@vger.kernel.org,
	Alex Williamson <alex.williamson@redhat.com>,
	David Woodhouse <dwmw2@infradead.org>,
	yi.y.sun@intel.com
Subject: Re: [RFC PATCH 2/4] iommu/vt-d: Add first level page table interfaces
Date: Tue, 8 Oct 2019 10:20:10 +0800	[thread overview]
Message-ID: <27232499-4f13-83c0-a1d3-e82e9183f3f0@linux.intel.com> (raw)
In-Reply-To: <20190929052532.GA12953@xz-x1>

Hi,

On 9/29/19 1:25 PM, Peter Xu wrote:
> On Sat, Sep 28, 2019 at 04:23:16PM +0800, Lu Baolu wrote:
>> Hi Peter,
>>
>> On 9/27/19 1:34 PM, Peter Xu wrote:
>>> Hi, Baolu,
>>>
>>> On Fri, Sep 27, 2019 at 10:27:24AM +0800, Lu Baolu wrote:
>>>>>>>> +	spin_lock(&(domain)->page_table_lock);				\
>>>>>>>
>>>>>>> Is this intended to lock here instead of taking the lock during the
>>>>>>> whole page table walk?  Is it safe?
>>>>>>>
>>>>>>> Taking the example where nm==PTE: when we reach here how do we
>>>>>>> guarantee that the PMD page that has this PTE is still valid?
>>>>>>
>>>>>> We will always keep the non-leaf pages in the table,
>>>>>
>>>>> I see.  Though, could I ask why?  It seems to me that the existing 2nd
>>>>> level page table does not keep these when unmap, and it's not even use
>>>>> locking at all by leveraging cmpxchg()?
>>>>
>>>> I still need some time to understand how cmpxchg() solves the race issue
>>>> when reclaims pages. For example.
>>>>
>>>> Thread A				Thread B
>>>> -A1: check all PTE's empty		-B1: up-level PDE valid
>>>> -A2: clear the up-level PDE
>>>> -A3: reclaim the page			-B2: populate the PTEs
>>>>
>>>> Both (A1,A2) and (B1,B2) should be atomic. Otherwise, race could happen.
>>>
>>> I'm not sure of this, but IMHO it is similarly because we need to
>>> allocate the iova ranges from iova allocator first, so thread A (who's
>>> going to unmap pages) and thread B (who's going to map new pages)
>>> should never have collapsed regions if happening concurrently.  I'm
>>
>> Although they don't collapse, they might share a same pmd entry. If A
>> cleared the pmd entry and B goes ahead with populating the pte's. It
>> will crash.
> 
> My understanding is that if A was not owning all the pages on that PMD
> entry then it will never free the page that was backing that PMD
> entry.  Please refer to the code in dma_pte_clear_level() where it
> has:
> 
>          /* If range covers entire pagetable, free it */
>          if (start_pfn <= level_pfn &&
>                  last_pfn >= level_pfn + level_size(level) - 1) {
>                  ...
>          } else {
>                  ...
>          }
> 
> Note that when going into the else block, the PMD won't be freed but
> only the PTEs that upon the PMD will be cleared.

Exactly! Thanks for pointing this out.

I will do the same thing in v2.

> 
> In the case you mentioned above, IMHO it should go into that else
> block.  Say, thread A must not contain the whole range of that PMD
> otherwise thread B won't get allocated with pages within that range
> covered by the same PMD.
> 
> Thanks,
> 

Best regards,
Baolu
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