On Tue 23-11-21 09:56:41, Suren Baghdasaryan wrote: > On Tue, Nov 23, 2021 at 5:19 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > On Tue, Nov 16, 2021 at 01:57:14PM -0800, Suren Baghdasaryan wrote: > > > @@ -3170,6 +3172,7 @@ void exit_mmap(struct mm_struct *mm) > > > unmap_vmas(&tlb, vma, 0, -1); > > > free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING); > > > tlb_finish_mmu(&tlb); > > > + mmap_write_unlock(mm); > > > > > > /* > > > * Walk the list again, actually closing and freeing it, > > > > Is there a reason to unlock here instead of after the remove_vma loop? > > We'll need the mmap sem held during that loop when VMAs are stored in > > the maple tree. > > I didn't realize remove_vma() would need to be protected as well. I > think I can move mmap_write_unlock down to cover the last walk too > with no impact. > Does anyone know if there was any specific reason to perform that last > walk with no locks held (as the comment states)? I can track that > comment back to Linux-2.6.12-rc2 merge with no earlier history, so not > sure if it's critical not to hold any locks at this point. Seems to me > it's ok to hold mmap_write_unlock but maybe I'm missing something? I suspect the primary reason was that neither fput (and callbacks invoked from it) nor vm_close would need to be very careful about interacting with mm locks. fput is async these days so it shouldn't be problematic. vm_ops->close doesn't have any real contract definition AFAIK but taking mmap_sem from those would be really suprising. They should be mostly destructing internal vma state and that shouldn't really require address space protection. -- Michal Hocko SUSE Labs
