Commits on Apr 3, 2020

1. Use vn_io_fault_uiomove on FreeBSD to avoid potential deadlock …

   on page fault.
   
   The comment describing vn_io_fault_uiomove:
   /*
    * Helper function to perform the requested uiomove operation using
    * the held pages for io->uio_iov[0].iov_base buffer instead of
    * copyin/copyout.  Access to the pages with uiomove_fromphys()
    * instead of iov_base prevents page faults that could occur due to
    * pmap_collect() invalidating the mapping created by
    * vm_fault_quick_hold_pages(), or pageout daemon, page laundry or
    * object cleanup revoking the write access from page mappings.
    *
    * Filesystems specified MNTK_NO_IOPF shall use vn_io_fault_uiomove()
    * instead of plain uiomove().
    */
   
   This used for vn_io_fault which has the following motivation:
   /*
    * The vn_io_fault() is a wrapper around vn_read() and vn_write() to
    * prevent the following deadlock:
    *
    * Assume that the thread A reads from the vnode vp1 into userspace
    * buffer buf1 backed by the pages of vnode vp2.  If a page in buf1 is
    * currently not resident, then system ends up with the call chain
    *   vn_read() -> VOP_READ(vp1) -> uiomove() -> [Page Fault] ->
    *     vm_fault(buf1) -> vnode_pager_getpages(vp2) -> VOP_GETPAGES(vp2)
    * which establishes lock order vp1->vn_lock, then vp2->vn_lock.
    * If, at the same time, thread B reads from vnode vp2 into buffer buf2
    * backed by the pages of vnode vp1, and some page in buf2 is not
    * resident, we get a reversed order vp2->vn_lock, then vp1->vn_lock.
    *
    * To prevent the lock order reversal and deadlock, vn_io_fault() does
    * not allow page faults to happen during VOP_READ() or VOP_WRITE().
    * Instead, it first tries to do the whole range i/o with pagefaults
    * disabled. If all pages in the i/o buffer are resident and mapped,
    * VOP will succeed (ignoring the genuine filesystem errors).
    * Otherwise, we get back EFAULT, and vn_io_fault() falls back to do
    * i/o in chunks, with all pages in the chunk prefaulted and held
    * using vm_fault_quick_hold_pages().
    *
    * Filesystems using this deadlock avoidance scheme should use the
    * array of the held pages from uio, saved in the curthread->td_ma,
    * instead of doing uiomove().  A helper function
    * vn_io_fault_uiomove() converts uiomove request into
    * uiomove_fromphys() over td_ma array.
    *
    * Since vnode locks do not cover the whole i/o anymore, rangelocks
    * make the current i/o request atomic with respect to other i/os and
    * truncations.
    */
   
   Signed-off-by: Matt Macy <mmacy@FreeBSD.org>

   

   mattmacy committed Apr 3, 2020
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