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    SubjectRe: [PATCH] mm: SLAB Out-of-memory diagnostics
    On Wed, Feb 29, 2012 at 09:37:23AM +0200, Pekka Enberg wrote:
    > On Wed, 29 Feb 2012, Rafael Aquini wrote:
    > > Following the example at mm/slub.c, add out-of-memory diagnostics to the SLAB
    > > allocator to help on debugging OOM conditions. This patch also adds a new
    > > sysctl, 'oom_dump_slabs_forced', that overrides the effect of __GFP_NOWARN page
    > > allocation flag and forces the kernel to report every slab allocation failure.
    > >
    > > An example print out looks like this:
    > >
    > > <snip page allocator out-of-memory message>
    > > SLAB: Unable to allocate memory on node 0 (gfp=0x11200)
    > > cache: bio-0, object size: 192, order: 0
    > > node0: slabs: 3/3, objs: 60/60, free: 0
    > >
    > > Signed-off-by: Rafael Aquini <aquini@redhat.com>
    > > ---
    > > Documentation/sysctl/vm.txt | 23 ++++++++++++++++++
    > > include/linux/slab.h | 2 +
    > > kernel/sysctl.c | 9 +++++++
    > > mm/slab.c | 55 ++++++++++++++++++++++++++++++++++++++++++-
    > > 4 files changed, 88 insertions(+), 1 deletions(-)
    >
    > No SLUB support for this?
    >

    SLUB already has its version of slab_out_of_memory. I did not propose the sysctl
    knob for slub, however. (If we find the knob useful, I can propose its extention
    to slub, later).

    > > diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
    > > index 96f0ee8..75bdf91 100644
    > > --- a/Documentation/sysctl/vm.txt
    > > +++ b/Documentation/sysctl/vm.txt
    > > @@ -498,6 +498,29 @@ this is causing problems for your system/application.
    > >
    > > ==============================================================
    > >
    > > +oom_dump_slabs_forced
    > > +
    > > +Overrides the effects of __GFP_NOWARN page allocation flag, thus forcing
    > > +the system to print warnings about every allocation failure for the
    > > +slab allocator, and helping on debugging certain OOM conditions.
    > > +The print out is pretty similar, and complements data that is reported by
    > > +the page allocator out-of-memory warning:
    > > +
    > > +<snip page allocator out-of-memory message>
    > > + SLAB: Unable to allocate memory on node 0 (gfp=0x11200)
    > > + cache: bio-0, object size: 192, order: 0
    > > + node0: slabs: 3/3, objs: 60/60, free: 0
    > > +
    > > +If this is set to zero, the default behavior is observed and warnings will only
    > > +be printed out for allocation requests that didn't set the __GFP_NOWARN flag.
    > > +
    > > +When set to non-zero, this information is shown whenever the allocator finds
    > > +itself failing to grant a request, regardless the __GFP_NOWARN flag status.
    > > +
    > > +The default value is 0 (disabled).
    > > +
    > > +==============================================================
    > > +
    >
    > Why do you want to add a sysctl for this? That'd be an ABI that we need to
    > keep around forever.
    >
    > Is there any reason we shouldn't just enable this unconditionally?

    I was afraid of this code becoming a source of garrulous and scaring warnings
    by just ignoring __GFP_NOWARN flag, however, I was also concerned with the
    'hiding' effect the flag imposes for certain requests, specially when one is
    interested in checking all those requests out. Therefore, I thought a sysctl
    knob would be the best option to control the __GFP_NOWARN overriding behavior of
    slab_out_of_memory printouts without messing with the allocation flags
    themselves, as well as not imposing the need for reboots to start checking all
    slab allocation failures out.

    Rafael


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