Messages in this thread | | | Subject | Re: [RFC PATCH 2/4] iommu/vt-d: Add first level page table interfaces | From | Lu Baolu <> | Date | Tue, 8 Oct 2019 10:20:10 +0800 |
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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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