Re: [shmem] 7a7256d5f5: WARNING:possible_recursive_locking_detected

[Ira Weiny <ira.weiny@intel.com>]

On Fri, Oct 21, 2022 at 01:30:41PM -0700, Andrew Morton wrote:
> On Fri, 21 Oct 2022 14:09:16 +0100 Matthew Wilcox <willy@infradead.org> wrote:
> 
> > On Fri, Oct 21, 2022 at 12:10:17PM +0800, kernel test robot wrote:
> > > FYI, we noticed WARNING:possible_recursive_locking_detected due to commit (built with gcc-11):
> > > 
> > > commit: 7a7256d5f512b6c17957df7f59cf5e281b3ddba3 ("shmem: convert shmem_mfill_atomic_pte() to use a folio")
> > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> > 
> > Ummm.  Looks to me like this now occurs because of this part of the
> > change:
> > 
> >                 if (!zeropage) {        /* COPY */
> > -                       page_kaddr = kmap_atomic(page);
> > +                       page_kaddr = kmap_local_folio(folio, 0);
> >                         ret = copy_from_user(page_kaddr,
> >                                              (const void __user *)src_addr,
> >                                              PAGE_SIZE);
> > -                       kunmap_atomic(page_kaddr);
> > +                       kunmap_local(page_kaddr);
> > 
> > Should I be using __copy_from_user_inatomic() here?

I would say not.  I'm curious why copy_from_user() was safe (at least did not
fail the checkers).  :-/

> 
> Caller __mcopy_atomic() is holding mmap_read_lock(dst_mm) and this
> copy_from_user() calls
> might_fault()->might_lock_read(current->mm->mmap_lock).
> 
> And I guess might_lock_read() gets upset because we're holding another
> mm's mmap_lock.  Which sounds OK to me, unless a concurrent
> mmap_write_lock() could jam things up.
> 
> But I cannot see why your patch would suddenly trigger this warning -
> kmap_local_folio() and kmap_atomic() are basically the same thing.

It is related to your patch but I think what you did made sense on the surface.

On the surface copy_from_user() should not require pagefaults to be disabled.
But that side affect of kmap_atomic() was being used here because it looks like
the code is designed to fallback if the fault was not allowed:[1]

mm/shmem.c
...
                        page_kaddr = kmap_local_folio(folio, 0);
                        ret = copy_from_user(page_kaddr,
                                             (const void __user *)src_addr,
                                             PAGE_SIZE);
                        kunmap_local(page_kaddr);

                        /* fallback to copy_from_user outside mmap_lock */
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                        if (unlikely(ret)) {
                                *pagep = &folio->page;
                                ret = -ENOENT;
                                /* don't free the page */
                                goto out_unacct_blocks;
                        }
...

So this is one of those rare places where the kmap_atomic() side effects were
being depended on...  :-(

[1] might_fault() does not actually mean the code completes the fault.

mm/memory.c
...
void __might_fault(const char *file, int line)
{
        if (pagefault_disabled())
                return;
...

> 
> I see that __mcopy_atomic() is using plain old kmap(), perhaps to work
> around this?  But that's 2015 code and I'm not sure we had such
> detailed lock checking in those days.

No kmap() can't work around this.  That works because the lock is released just
above that.

mm/userfaultfd.c
...
                        mmap_read_unlock(dst_mm);
                        BUG_ON(!page);

                        page_kaddr = kmap(page);
                        err = copy_from_user(page_kaddr,
                                             (const void __user *) src_addr,
                                             PAGE_SIZE);
                        kunmap(page);
...

So I think the correct solution is below because we want to prevent the page
fault.

Ira

diff --git a/mm/shmem.c b/mm/shmem.c
index 8280a5cb48df..6c8e99bf5983 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2424,9 +2424,11 @@ int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,

                if (!zeropage) {        /* COPY */
                        page_kaddr = kmap_local_folio(folio, 0);
+                       pagefault_disable()
                        ret = copy_from_user(page_kaddr,
                                             (const void __user *)src_addr,
                                             PAGE_SIZE);
+                       pagefault_enable()
                        kunmap_local(page_kaddr);

                        /* fallback to copy_from_user outside mmap_lock */