lkml.org 
[lkml]   [2015]   [Jan]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    SubjectRe: Linux 3.19-rc3
    From
    On Fri, Jan 9, 2015 at 7:29 PM, Laszlo Ersek <lersek@redhat.com> wrote:
    >
    > I've bisected this issue to

    .. commit f045bbb9fa1b ("mmu_gather: fix over-eager
    tlb_flush_mmu_free() calling")

    Hmm. That commit literally just undoes something that commit
    fb7332a9fedf ("mmu_gather: move minimal range calculations into
    generic code") changed, and that was very wrong on x86.

    But arm64 did have very different TLB flushing logic, so there may be
    some ARM64 reason that Will did that change originally, and then he
    forgot that reason when he ack'ed commit f045bbb9fa1b that undid it.

    Will?

    Before your mmu_gather range calculations commit, we used to have

    In tlb_flush_mmu_tlbonly():

    tlb->need_flush = 0;

    and in tlb_flush_mmu():

    if (!tlb->need_flush)
    return;

    and your commit changed the rule to be

    !tlb->need_flush == !tlb->end

    so in the current tree we have

    In tlb_flush_mmu_tlbonly():

    __tlb_reset_range(tlb); // replaces "tlb->need_flush = 0;"

    and in tlb_flush_mmu():

    if (!tlb->end) // replaces if (!tlb->need_flush)
    return;

    so we seem to do exactly the same as 3.18.

    But in your original patch, you moved that "if (!tlb->end) return;"
    check from tlb_flush_mmu() into tlb_flush_mmu_tlbonly(), and that
    apparently is actually needed on arm64. But *why*?

    Also, looking at that commit fb7332a9fedf, I note that some of the
    "need_flush" setting was simply removed. See for example
    arch/powerpc/mm/hugetlbpage.c, and also in mm/memory.c:
    tlb_remove_table(). Is there something non-obvious that sets tlb->end
    there?

    The other need_flush removals seem to all be paired with adding a
    __tlb_adjust_range() call, which will set ->end.

    I'm starting to suspect that you moved the need_flush test into
    tlbonly exactly because you removed that

    tlb->need_flush = 1;

    from mm/memory.c: tlb_remove_table().

    x86 doesn't care, because x86 doesn't *use* tlb_remove_table(). But
    arm64 does, at least with the RCU freeing.

    Any ideas?

    Linus


    \
     
     \ /
      Last update: 2015-01-10 06:01    [W:4.113 / U:0.040 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site