On 2024/3/29 23:54, Thorvald Natvig wrote: > Did this patch (or another fix for the same problem) make it through? I'm sorry but I didn't have enough time to figure out a complete solution and this stuff got lost in my mind... > If not, is there anything we can do to help? I would try to send a formal patch as soon as possible. A full test and review would be really helpful. Thanks. > > - Thorvald > > > On Sat, Feb 3, 2024 at 5:54 PM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote: >> >> 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 >>>>> >>>>> . >>>>> >> > . >
