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; root@qemu:~# ./hugetlb_vma_lock Clean exit > > Thanks. >
