Problems with use of M_NOWWAIT in ATA

[Matthew Dillon]

    Soren, while fixing some issues in DFly related to the ATA driver I
    found a serious problem in your driver... actually, it appears to be
    in ata-ng -stable and -current as well.

    The problem is that you are using M_NOWAIT all over the place.  M_NOWAIT
    allows malloc() to fail if/when resources are temporarily exhausted.
    In DFly this can occur more often because it causes malloc() to return
    NULL if cannot lock the kernel_map, but I believe malloc() can return
    NULL in -current and -stable as well (e.g. if the VM page free list is
    empty).

    Also, at least in the 4.8 ATA driver, the driver tries to revert to PIO
    mode if it cannot allocate a DMA buffer.  This is bad because PIO
    might not necessarily work.  It just isn't appropriate to revert to PIO
    mode under any circumstances other then a hardware DMA failure.  In
    -current you are using bus dma tags and I don't know whether that solves
    the issue, but even in -CURRENT you are allocating request structures
    with M_NOWAIT (in ata_alloc_request()) and that is just plain not right.  
    You are virtually guarenteed to corrupt any filesystem trying to issue
    a write for which ata_alloc_request() fails.  It *CANNOT* fail.

    I know the problem is complicated somewhat by the fact that you cannot
    simply replace that code with a blocking wait without creating a deadlock
    situation.  What you really need to do is create a request pipeline and
    block up-front to wait for an I/O operation to complete which frees up
    an existing request (e.g. before getting the tag).  If you do not want
    to block in ad_start() you can place the original io buffer on a 
    queue and reissue when requests/DMA memory are freed up by completing
    I/O's.

    I am going to be hacking this up in DFly and I'll email you a patch set
    when I'm done.  This is just a head's up that there are some serious
    issues in the ATA driver code related to memory allocation.

						-Matt