On Sat, Dec 09, 2023 at 09:18:42PM -0800, Hugh Dickins wrote: > syzbot reports oops in lockdep's __lock_acquire(), called from > __pte_offset_map_lock() called from filemap_map_pages(); or when I run the > repro, the oops comes in pmd_install(), called from filemap_map_pmd() > called from filemap_map_pages(), just before the __pte_offset_map_lock(). > > The problem is that filemap_map_pmd() has been assuming that when it finds > pmd_none(), a page table has already been prepared in prealloc_pte; and > indeed do_fault_around() has been careful to preallocate one there, when > it finds pmd_none(): but what if *pmd became none in between? > > My 6.6 mods in mm/khugepaged.c, avoiding mmap_lock for write, have made it > easy for *pmd to be cleared while servicing a page fault; but even before > those, a huge *pmd might be zapped while a fault is serviced. > > The difference in symptomatic stack traces comes from the "memory model" > in use: pmd_install() uses pmd_populate() uses page_to_pfn(): in some > models that is strict, and will oops on the NULL prealloc_pte; in other > models, it will construct a bogus value to be populated into *pmd, then > __pte_offset_map_lock() oops when trying to access split ptlock pointer > (or some other symptom in normal case of ptlock embedded not pointer). > > Link: https://lore.kernel.org/linux-mm/20231115065506.19780-1-jose.pekkarinen@xxxxxxxxxxx/ > Link: https://lkml.kernel.org/r/6ed0c50c-78ef-0719-b3c5-60c0c010431c@xxxxxxxxxx > Fixes: f9ce0be71d1f ("mm: Cleanup faultaround and finish_fault() codepaths") > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> > Reported-and-tested-by: syzbot+89edd67979b52675ddec@xxxxxxxxxxxxxxxxxxxxxxxxx > Closes: https://lore.kernel.org/linux-mm/0000000000005e44550608a0806c@xxxxxxxxxx/ > Reviewed-by: David Hildenbrand <david@xxxxxxxxxx> > Cc: Jann Horn <jannh@xxxxxxxxxx>, > Cc: José Pekkarinen <jose.pekkarinen@xxxxxxxxxxx> > Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> > Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> [5.12+] > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > (cherry picked from commit 9aa1345d66b8132745ffb99b348b1492088da9e2) > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> > --- > mm/filemap.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > Now queued up, thanks. greg k-h
