On Mon, Oct 24, 2022 at 09:54:30AM -0700, Ira Weiny wrote: > On Sun, Oct 23, 2022 at 09:33:05PM -0700, Ira wrote: > > From: Ira Weiny <ira.weiny@xxxxxxxxx> > > > > The kernel test robot flagged a recursive lock as a result of a > > conversion from kmap_atomic() to kmap_local_folio()[Link] > > > > The cause was due to the code depending on the kmap_atomic() side effect > > of disabling page faults. In that case the code expects the fault to > > fail and take the fallback case. > > > > git archaeology implied that the recursion may not be an actual bug.[1] > > However, the mmap_lock needed in the fault may be the one held.[2] > > > > Add an explicit pagefault_disable() and a big comment to explain this > > for future souls looking at this code. > > > > [1] https://lore.kernel.org/all/Y1MymJ%2FINb45AdaY@iweiny-desk3/ > > [2] https://lore.kernel.org/all/Y1M2p9OtBGnKwGUE@x1n/ > > > > Fixes: 7a7256d5f512 ("shmem: convert shmem_mfill_atomic_pte() to use a folio") > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > > Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx> > > Cc: Peter Xu <peterx@xxxxxxxxxx> > > Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> > > Reported-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> > > Reported-by: kernel test robot <yujie.liu@xxxxxxxxx> > > Link: https://lore.kernel.org/r/202210211215.9dc6efb5-yujie.liu@xxxxxxxxx > > Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx> > > > > --- > > Thanks to Matt and Andrew for initial diagnosis. > > Thanks to Randy for pointing out C code needs ';' :-D > > Thanks to Andrew for suggesting an elaborate comment > > Thanks to Peter for pointing out that the mm's may be the same. > > --- > > mm/shmem.c | 7 +++++++ > > 1 file changed, 7 insertions(+) > > > > diff --git a/mm/shmem.c b/mm/shmem.c > > index 8280a5cb48df..c1bca31cd485 100644 > > --- a/mm/shmem.c > > +++ b/mm/shmem.c > > @@ -2424,9 +2424,16 @@ int shmem_mfill_atomic_pte(struct mm_struct *dst_mm, > > > > if (!zeropage) { /* COPY */ > > page_kaddr = kmap_local_folio(folio, 0); > > + /* > > + * The mmap_lock is held here. Disable page faults to > > + * prevent deadlock should copy_from_user() fault. The > > + * copy will be retried outside the mmap_lock. > > + */ > > Offline Dave Hansen and I were discussing this and he was concerned that this > comment implies that a deadlock would always occur rather than might occur. > > I was not clear on this as I was thinking the read mmap_lock was non-recursive. > > So I think we have 3 cases only 1 of which will actually deadlock and is, as > Dave puts it, currently theoretical. > > 1) Different mm's are in play (no issue) > 2) Readlock implementation is recursive and same mm is in play (no issue) > 3) Readlock implementation is _not_ recursive (issue) > > In both 1 and 2 lockdep is incorrectly flagging the issue but 3 is a problem > and I think this is what Andrea was thinking. The readlock implementation is only recursive if nobody else has taken a write lock. AIUI, no other process can take a write lock on the mmap_lock (other processes can take read locks by examining /proc/$pid/maps, for example), although maybe ptrace can take the mmap_lock for write? But if you have a multithreaded process, one of the other threads can call mmap() and that will prevent recursion (due to fairness). Even if it's a different process that you're trying to acquire the mmap read lock on, you can still get into a deadly embrace. eg: process A thread 1 takes read lock on own mmap_lock process A thread 2 calls mmap, blocks taking write lock process B thread 1 takes page fault, read lock on own mmap lock process B thread 2 calls mmap, blocks taking write lock process A thread 1 blocks taking read lock on process B process B thread 1 blocks taking read lock on process A Now all four threads are blocked waiting for each other.
