lkml.org 
[lkml]   [2019]   [Oct]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [RFC PATCH 2/4] iommu/vt-d: Add first level page table interfaces
From
Date
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

\
 
 \ /
  Last update: 2019-10-08 04:23    [W:0.313 / U:0.116 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site