Ackerley Tng <ackerleytng@xxxxxxxxxx> writes:
> Yan Zhao <yan.y.zhao@xxxxxxxxx> writes:
>
>> On Fri, Mar 14, 2025 at 05:20:21PM +0800, Yan Zhao wrote:
>>> This patch would cause host deadlock when booting up a TDX VM even if huge page
>>> is turned off. I currently reverted this patch. No further debug yet.
>> This is because kvm_gmem_populate() takes filemap invalidation lock, and for
>> TDX, kvm_gmem_populate() further invokes kvm_gmem_get_pfn(), causing deadlock.
>>
>> kvm_gmem_populate
>> filemap_invalidate_lock
>> post_populate
>> tdx_gmem_post_populate
>> kvm_tdp_map_page
>> kvm_mmu_do_page_fault
>> kvm_tdp_page_fault
>> kvm_tdp_mmu_page_fault
>> kvm_mmu_faultin_pfn
>> __kvm_mmu_faultin_pfn
>> kvm_mmu_faultin_pfn_private
>> kvm_gmem_get_pfn
>> filemap_invalidate_lock_shared
>>
>> Though, kvm_gmem_populate() is able to take shared filemap invalidation lock,
>> (then no deadlock), lockdep would still warn "Possible unsafe locking scenario:
>> ...DEADLOCK" due to the recursive shared lock, since commit e918188611f0
>> ("locking: More accurate annotations for read_lock()").
>>
>
> Thank you for investigating. This should be fixed in the next revision.
>
This was not fixed in v2 [1], I misunderstood this locking issue.
IIUC kvm_gmem_populate() gets a pfn via __kvm_gmem_get_pfn(), then calls
part of the KVM fault handler to map the pfn into secure EPTs, then
calls the TDX module for the copy+encrypt.
Regarding this lock, seems like KVM'S MMU lock is already held while TDX
does the copy+encrypt. Why must the filemap_invalidate_lock() also be
held throughout the process?
If we don't have to hold the filemap_invalidate_lock() throughout,
1. Would it be possible to call kvm_gmem_get_pfn() to get the pfn
instead of calling __kvm_gmem_get_pfn() and managing the lock in a
loop?
2. Would it be possible to trigger the kvm fault path from
kvm_gmem_populate() so that we don't rebuild the get_pfn+mapping
logic and reuse the entire faulting code? That way the
filemap_invalidate_lock() will only be held while getting a pfn.
[1] https://lore.kernel.org/all/cover.1747264138.git.ackerleytng@xxxxxxxxxx/T/
>>> > @@ -819,12 +827,16 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
>>> > pgoff_t index = kvm_gmem_get_index(slot, gfn);
>>> > struct file *file = kvm_gmem_get_file(slot);
>>> > int max_order_local;
>>> > + struct address_space *mapping;
>>> > struct folio *folio;
>>> > int r = 0;
>>> >
>>> > if (!file)
>>> > return -EFAULT;
>>> >
>>> > + mapping = file->f_inode->i_mapping;
>>> > + filemap_invalidate_lock_shared(mapping);
>>> > +
>>> > /*
>>> > * The caller might pass a NULL 'max_order', but internally this
>>> > * function needs to be aware of any order limitations set by
>>> > @@ -838,6 +850,7 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
>>> > folio = __kvm_gmem_get_pfn(file, slot, index, pfn, &max_order_local);
>>> > if (IS_ERR(folio)) {
>>> > r = PTR_ERR(folio);
>>> > + filemap_invalidate_unlock_shared(mapping);
>>> > goto out;
>>> > }
>>> >
>>> > @@ -845,6 +858,7 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
>>> > r = kvm_gmem_prepare_folio(kvm, file, slot, gfn, folio, max_order_local);
>>> >
>>> > folio_unlock(folio);
>>> > + filemap_invalidate_unlock_shared(mapping);
>>> >
>>> > if (!r)
>>> > *page = folio_file_page(folio, index);
>>> > --
>>> > 2.25.1
>>> >
>>> >
