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    Subject[PATCH 11/15] ptwalk: copy_pte_range hang
    This patch is the odd-one-out of the sequence.  The one before adjusted
    copy_pte_range from a for loop to a do while loop, and it was therefore
    simplest to check for lockbreak before copying pte: possibility that it
    might keep getting preempted without making progress under some loads.

    Some loads such as startup: 2*HT*P4 with preemption cannot even reach
    multiuser login. Suspect needs_lockbreak is broken, can get in a state
    when it remains forever true. Investigate that later: for now, and for
    all time, it makes sense to aim for a little progress before breaking
    out; and we can manage more pte_nones than copies.

    Signed-off-by: Hugh Dickins <hugh@veritas.com>
    ---

    mm/memory.c | 11 ++++++++---
    1 files changed, 8 insertions(+), 3 deletions(-)

    --- ptwalk10/mm/memory.c 2005-03-09 01:38:12.000000000 +0000
    +++ ptwalk11/mm/memory.c 2005-03-09 01:38:54.000000000 +0000
    @@ -328,6 +328,7 @@ static int copy_pte_range(struct mm_stru
    {
    pte_t *src_pte, *dst_pte;
    unsigned long vm_flags = vma->vm_flags;
    + int progress;

    again:
    dst_pte = pte_alloc_map(dst_mm, dst_pmd, addr);
    @@ -335,19 +336,23 @@ again:
    return -ENOMEM;
    src_pte = pte_offset_map_nested(src_pmd, addr);

    + progress = 0;
    spin_lock(&src_mm->page_table_lock);
    do {
    /*
    * We are holding two locks at this point - either of them
    * could generate latencies in another task on another CPU.
    */
    - if (need_resched() ||
    + if (progress >= 32 && (need_resched() ||
    need_lockbreak(&src_mm->page_table_lock) ||
    - need_lockbreak(&dst_mm->page_table_lock))
    + need_lockbreak(&dst_mm->page_table_lock)))
    break;
    - if (pte_none(*src_pte))
    + if (pte_none(*src_pte)) {
    + progress++;
    continue;
    + }
    copy_one_pte(dst_mm, src_mm, dst_pte, src_pte, vm_flags, addr);
    + progress += 8;
    } while (dst_pte++, src_pte++, addr += PAGE_SIZE, addr != end);
    spin_unlock(&src_mm->page_table_lock);

    -
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