Fails to use JVM ballooning

[nozohono]

I just want to use Jvm Balloon to run applications and osv will hang when partial copy completes.I find that osv hangs because it cannot access vma_list_rwlock.write() .I guess that this may be caused by mprotect——because mprotect is the last function that accsess vma_list_rwlock.write().

[wkozaczuk]

This appears to be a duplicate of #398. @nyh can you confirm it?

[nyh]

@glommer ?

[glommer]

This seems like the same issue, yes. Unfortunately I don't have a lot of time to look into it these days

[nozohono]

I think I have found the reason.In the vm_fault function there are locked by vma_list_mutex.for_read()

osv/core/mmu.cc

Line 1326 in 4e425d4

void vm_fault(uintptr_t addr, exception_frame* ef)

osv/core/mmu.cc

Line 1335 in 4e425d4

WITH_LOCK(vma_list_mutex.for_read()) {

and in the scope of vma_list_mutex.for_read() the function jvm_balloon_fault will be called

osv/java/jvm/jvm_balloon.cc

Line 352 in 4e425d4

bool jvm_balloon_fault(balloon_ptr b, exception_frame *ef, mmu::jvm_balloon_vma *vma)

this time vma will be delete ,and the destructor will call map_anon(),which will access the vma_list_mutex.for_write().Apparently it cannot access this because the same thread hold read lock.

osv/core/mmu.cc

Line 1570 in 4e425d4

jvm_balloon_vma::~jvm_balloon_vma()

So I simply solve this problem by replacing the read lock with write lock and now I can use this function.I think there will be a nicer method

[wkozaczuk]

@nozohono Honestly I am not too familiar with this code (yet:-)). Which version of JVM (8, ..., 12) have you tested it with (I am assuming it worked for you)?

Have you seen these two commits - e69fa2b and 4bd8720. I wonder if they may help you validate your solution and makes sure it does not break other thinks.

[wkozaczuk]

I also think this issue is a duplicate of #796.

[wkozaczuk]

Linking #796 (comment) and #796 (comment) as useful information.