• Song Liu's avatar
    mm,thp: recheck each page before collapsing file THP · 4655e5e5
    Song Liu authored
    In collapse_file(), for !is_shmem case, current check cannot guarantee
    the locked page is up-to-date.  Specifically, xas_unlock_irq() should
    not be called before lock_page() and get_page(); and it is necessary to
    recheck PageUptodate() after locking the page.
    
    With this bug and CONFIG_READ_ONLY_THP_FOR_FS=y, madvise(HUGE)'ed .text
    may contain corrupted data.  This is because khugepaged mistakenly
    collapses some not up-to-date sub pages into a huge page, and assumes
    the huge page is up-to-date.  This will NOT corrupt data in the disk,
    because the page is read-only and never written back.  Fix this by
    properly checking PageUptodate() after locking the page.  This check
    replaces "VM_BUG_ON_PAGE(!PageUptodate(page), page);".
    
    Also, move PageDirty() check after locking the page.  Current khugepaged
    should not try to collapse dirty file THP, because it is limited to
    read-only .text.  The only case we hit a dirty page here is when the
    page hasn't been written since write.  Bail out and retry when this
    happens.
    
    syzbot reported bug on previous version of this patch.
    
    Link: http://lkml.kernel.org/r/20191106060930.2571389-2-songliubraving@fb.com
    Fixes: 99cb0dbd
    
     ("mm,thp: add read-only THP support for (non-shmem) FS")
    Signed-off-by: default avatarSong Liu <songliubraving@fb.com>
    Reported-by: syzbot+efb9e48b9fbdc49bb34a@syzkaller.appspotmail.com
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: William Kucharski <william.kucharski@oracle.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    4655e5e5