On 2024/2/3 5:02, Jane Chu wrote: > On 1/30/2024 10:51 PM, Miaohe Lin wrote: > >> On 2024/1/30 12:08, Liam R. Howlett wrote: >>> * Miaohe Lin <linmiaohe@xxxxxxxxxx> [240129 21:14]: >>>> On 2024/1/30 0:17, Liam R. Howlett wrote: >>>>> * Miaohe Lin <linmiaohe@xxxxxxxxxx> [240129 07:56]: >>>>>> On 2024/1/27 18:13, Miaohe Lin wrote: >>>>>>> On 2024/1/26 15:50, Muchun Song wrote: >>>>>>>>> On Jan 26, 2024, at 04:28, Thorvald Natvig <thorvald@xxxxxxxxxx> wrote: >>>>>>>>> >>>>>>>>> We've found what appears to be a lock issue that results in a blocked >>>>>>>>> process somewhere in hugetlbfs for shared maps; seemingly from an >>>>>>>>> interaction between hugetlb_vm_op_open and hugetlb_vmdelete_list. >>>>>>>>> >>>>>>>>> Based on some added pr_warn, we believe the following is happening: >>>>>>>>> When hugetlb_vmdelete_list is entered from the child process, >>>>>>>>> vma->vm_private_data is NULL, and hence hugetlb_vma_trylock_write does >>>>>>>>> not lock, since neither __vma_shareable_lock nor __vma_private_lock >>>>>>>>> are true. >>>>>>>>> >>>>>>>>> While hugetlb_vmdelete_list is executing, the parent process does >>>>>>>>> fork(), which ends up in hugetlb_vm_op_open, which in turn allocates a >>>>>>>>> lock for the same vma. >>>>>>>>> >>>>>>>>> Thus, when the hugetlb_vmdelete_list in the child reaches the end of >>>>>>>>> the function, vma->vm_private_data is now populated, and hence >>>>>>>>> hugetlb_vma_unlock_write tries to unlock the vma_lock, which it does >>>>>>>>> not hold. >>>>>>>> Thanks for your report. ->vm_private_data was introduced since the >>>>>>>> series [1]. So I suspect it was caused by this. But I haven't reviewed >>>>>>>> that at that time (actually, it is a little complex in pmd sharing >>>>>>>> case). I saw Miaohe had reviewed many of those. >>>>>>>> >>>>>>>> CC Miaohe, maybe he has some ideas on this. >>>>>>>> >>>>>>>> [1] https://lore.kernel.org/all/20220914221810.95771-7-mike.kravetz@xxxxxxxxxx/T/#m2141e4bc30401a8ce490b1965b9bad74e7f791ff >>>>>>>> >>>>>>>> Thanks. >>>>>>>> >>>>>>>>> dmesg: >>>>>>>>> WARNING: bad unlock balance detected! >>>>>>>>> 6.8.0-rc1+ #24 Not tainted >>>>>>>>> ------------------------------------- >>>>>>>>> lock/2613 is trying to release lock (&vma_lock->rw_sema) at: >>>>>>>>> [<ffffffffa94c6128>] hugetlb_vma_unlock_write+0x48/0x60 >>>>>>>>> but there are no more locks to release! >>>>>>> Thanks for your report. It seems there's a race: >>>>>>> >>>>>>> CPU 1 CPU 2 >>>>>>> fork hugetlbfs_fallocate >>>>>>> dup_mmap hugetlbfs_punch_hole >>>>>>> i_mmap_lock_write(mapping); >>>>>>> vma_interval_tree_insert_after -- Child vma is visible through i_mmap tree. >>>>>>> i_mmap_unlock_write(mapping); >>>>>>> hugetlb_dup_vma_private -- Clear vma_lock outside i_mmap_rwsem! i_mmap_lock_write(mapping); >>>>>>> hugetlb_vmdelete_list >>>>>>> vma_interval_tree_foreach >>>>>>> hugetlb_vma_trylock_write -- Vma_lock is cleared. >>>>>>> tmp->vm_ops->open -- Alloc new vma_lock outside i_mmap_rwsem! >>>>>>> hugetlb_vma_unlock_write -- Vma_lock is assigned!!! >>>>>>> i_mmap_unlock_write(mapping); >>>>>>> >>>>>>> hugetlb_dup_vma_private and hugetlb_vm_op_open are called outside i_mmap_rwsem lock. So there will be another bugs behind it. >>>>>>> But I'm not really sure. I will take a more closed look at next week. >>>>>> This can be fixed by deferring vma_interval_tree_insert_after() until vma is fully initialized. >>>>>> But I'm not sure whether there're side effects with this patch. >>>>>> >>>>>> linux-UJMmTI:/home/linmiaohe/mm # git diff >>>>>> diff --git a/kernel/fork.c b/kernel/fork.c >>>>>> index 47ff3b35352e..2ef2711452e0 100644 >>>>>> --- a/kernel/fork.c >>>>>> +++ b/kernel/fork.c >>>>>> @@ -712,21 +712,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, >>>>>> } else if (anon_vma_fork(tmp, mpnt)) >>>>>> goto fail_nomem_anon_vma_fork; >>>>>> vm_flags_clear(tmp, VM_LOCKED_MASK); >>>>>> - file = tmp->vm_file; >>>>>> - if (file) { >>>>>> - struct address_space *mapping = file->f_mapping; >>>>>> - >>>>>> - get_file(file); >>>>>> - i_mmap_lock_write(mapping); >>>>>> - if (vma_is_shared_maywrite(tmp)) >>>>>> - mapping_allow_writable(mapping); >>>>>> - flush_dcache_mmap_lock(mapping); >>>>>> - /* insert tmp into the share list, just after mpnt */ >>>>>> - vma_interval_tree_insert_after(tmp, mpnt, >>>>>> - &mapping->i_mmap); >>>>>> - flush_dcache_mmap_unlock(mapping); >>>>>> - i_mmap_unlock_write(mapping); >>>>>> - } >>>>>> >>>>>> /* >>>>>> * Copy/update hugetlb private vma information. >>>>>> @@ -747,6 +732,22 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, >>>>>> if (tmp->vm_ops && tmp->vm_ops->open) >>>>>> tmp->vm_ops->open(tmp); >>>>>> >>>>>> + file = tmp->vm_file; >>>>>> + if (file) { >>>>>> + struct address_space *mapping = file->f_mapping; >>>>>> + >>>>>> + get_file(file); >>>>>> + i_mmap_lock_write(mapping); >>>>>> + if (vma_is_shared_maywrite(tmp)) >>>>>> + mapping_allow_writable(mapping); >>>>>> + flush_dcache_mmap_lock(mapping); >>>>>> + /* insert tmp into the share list, just after mpnt. */ >>>>>> + vma_interval_tree_insert_after(tmp, mpnt, >>>>>> + &mapping->i_mmap); >>>>>> + flush_dcache_mmap_unlock(mapping); >>>>>> + i_mmap_unlock_write(mapping); >>>>>> + } >>>>>> + >>>>>> if (retval) { >>>>>> mpnt = vma_next(&vmi); >>>>>> goto loop_out; >>>>>> >>>>>> >>>>> How is this possible? I thought, as specified in mm/rmap.c, that the >>>>> hugetlbfs path would be holding the mmap lock (which is also held in the >>>>> fork path)? >>>> The fork path holds the mmap lock from parent A and other childs(except first child B) while hugetlbfs path >>>> holds the mmap lock from first child B. So the mmap lock won't help here because it comes from different mm. >>>> Or am I miss something? >>> You are correct. It is also in mm/rmap.c: >>> * hugetlbfs PageHuge() take locks in this order: >>> * hugetlb_fault_mutex (hugetlbfs specific page fault mutex) >>> * vma_lock (hugetlb specific lock for pmd_sharing) >>> * mapping->i_mmap_rwsem (also used for hugetlb pmd sharing) >>> * page->flags PG_locked (lock_page) >>> >>> Does it make sense for hugetlb_dup_vma_private() to assert >>> mapping->i_mmap_rwsem is locked? When is that necessary? >> I'm afraid not. AFAICS, vma_lock(vma->vm_private_data) is only modified at the time of >> vma creating or destroy. Vma_lock is not supposed to be used at that time. >> >>> I also think it might be safer to move the hugetlb_dup_vma_private() >>> call up instead of the insert into the interval tree down? >>> See the following comment from mmap.c: >>> >>> /* >>> * Put into interval tree now, so instantiated pages >>> * are visible to arm/parisc __flush_dcache_page >>> * throughout; but we cannot insert into address >>> * space until vma start or end is updated. >>> */ >>> >>> So there may be arch dependent reasons for this order. >> Yes, it should be safer to move hugetlb_dup_vma_private() call up. But we also need to move tmp->vm_ops->open(tmp) call up. >> Or the race still exists: >> >> CPU 1 CPU 2 >> fork hugetlbfs_fallocate >> dup_mmap hugetlbfs_punch_hole >> hugetlb_dup_vma_private -- Clear vma_lock. <-- it is moved up. >> i_mmap_lock_write(mapping); >> vma_interval_tree_insert_after -- Child vma is visible through i_mmap tree. >> i_mmap_unlock_write(mapping); >> i_mmap_lock_write(mapping); >> hugetlb_vmdelete_list >> vma_interval_tree_foreach >> hugetlb_vma_trylock_write -- Vma_lock is already cleared. >> tmp->vm_ops->open -- Alloc new vma_lock outside i_mmap_rwsem! >> hugetlb_vma_unlock_write -- Vma_lock is assigned!!! >> i_mmap_unlock_write(mapping); >> >> >> My patch should not be a complete solution. It's used to prove and fix the race quickly. It's very great if you or >> someone else can provide a better and safer solution. > > But, your patch has already moved the vma_interval_tree_insert_after() block after the > > tmp->vm_ops->open(tmp) call, right? Hence, there should be no more race with truncation? Sure. There won't be more race if tmp->vm_ops->open(tmp) call is *also* moved above vma_interval_tree_insert_after() block. But I'm not sure it's safe to do so. There might be some obscure assumptions about the time to call vma_interval_tree_insert_after(). Thanks. > > thanks, > -jane > >> Thanks. >> >>> Thanks, >>> Liam >>> >>> . >>>
