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    SubjectRe: [PATCH 07/19] mm/mpol: Add MPOL_MF_NOOP
    On Tue, Jul 31, 2012 at 09:12:11PM +0200, Peter Zijlstra wrote:
    > From: Lee Schermerhorn <lee.schermerhorn@hp.com>
    >
    > This patch augments the MPOL_MF_LAZY feature by adding a "NOOP"
    > policy to mbind(). When the NOOP policy is used with the 'MOVE
    > and 'LAZY flags, mbind() [check_range()] will walk the specified
    > range and unmap eligible pages so that they will be migrated on
    > next touch.
    >
    > This allows an application to prepare for a new phase of operation
    > where different regions of shared storage will be assigned to
    > worker threads, w/o changing policy. Note that we could just use
    > "default" policy in this case. However, this also allows an
    > application to request that pages be migrated, only if necessary,
    > to follow any arbitrary policy that might currently apply to a
    > range of pages, without knowing the policy, or without specifying
    > multiple mbind()s for ranges with different policies.

    This is a new kapi change. I could hardly understand the above so I
    wonder how long it will take before userland programmers will be
    familiar with MPOL_NOOP to actually use it in most apps? Could you
    just enable/disable your logics using a sysfs knob instead?

    enabling/disabling sched-numa is something an admin can easily do with
    a sysfs control, patching and rebuilding a proprietary app using mbind
    calls, no way, especially if the app is proprietary.

    >
    > Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
    > Cc: Rik van Riel <riel@redhat.com>
    > Cc: Andrew Morton <akpm@linux-foundation.org>
    > Cc: Linus Torvalds <torvalds@linux-foundation.org>
    > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    > ---
    > include/linux/mempolicy.h | 1 +
    > mm/mempolicy.c | 8 ++++----
    > 2 files changed, 5 insertions(+), 4 deletions(-)
    >
    > diff --git a/include/linux/mempolicy.h b/include/linux/mempolicy.h
    > index 87fabfa..668311a 100644
    > --- a/include/linux/mempolicy.h
    > +++ b/include/linux/mempolicy.h
    > @@ -21,6 +21,7 @@ enum {
    > MPOL_BIND,
    > MPOL_INTERLEAVE,
    > MPOL_LOCAL,
    > + MPOL_NOOP, /* retain existing policy for range */
    > MPOL_MAX, /* always last member of enum */
    > };
    >
    > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
    > index 4fba5f2..251ef31 100644
    > --- a/mm/mempolicy.c
    > +++ b/mm/mempolicy.c
    > @@ -251,10 +251,10 @@ static struct mempolicy *mpol_new(unsigned short mode, unsigned short flags,
    > pr_debug("setting mode %d flags %d nodes[0] %lx\n",
    > mode, flags, nodes ? nodes_addr(*nodes)[0] : -1);
    >
    > - if (mode == MPOL_DEFAULT) {
    > + if (mode == MPOL_DEFAULT || mode == MPOL_NOOP) {
    > if (nodes && !nodes_empty(*nodes))
    > return ERR_PTR(-EINVAL);
    > - return NULL; /* simply delete any existing policy */
    > + return NULL;
    > }
    > VM_BUG_ON(!nodes);
    >
    > @@ -1069,7 +1069,7 @@ static long do_mbind(unsigned long start, unsigned long len,
    > if (start & ~PAGE_MASK)
    > return -EINVAL;
    >
    > - if (mode == MPOL_DEFAULT)
    > + if (mode == MPOL_DEFAULT || mode == MPOL_NOOP)
    > flags &= ~MPOL_MF_STRICT;
    >
    > len = (len + PAGE_SIZE - 1) & PAGE_MASK;
    > @@ -1121,7 +1121,7 @@ static long do_mbind(unsigned long start, unsigned long len,
    > flags | MPOL_MF_INVERT, &pagelist);
    >
    > err = PTR_ERR(vma); /* maybe ... */
    > - if (!IS_ERR(vma))
    > + if (!IS_ERR(vma) && mode != MPOL_NOOP)
    > err = mbind_range(mm, start, end, new);
    >
    > if (!err) {
    > --
    > 1.7.2.3
    >
    >
    >
    > --
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