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    SubjectRe: [PATCH 2/3] mm, oom: do not enfore OOM killer for __GFP_NOFAIL automatically
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    On Wednesday, January 25, 2017 4:00 PM Michal Hocko wrote: 
    > On Wed 25-01-17 15:00:51, Hillf Danton wrote:
    > > On Tuesday, January 24, 2017 8:41 PM Michal Hocko wrote:
    > > > On Fri 20-01-17 16:33:36, Hillf Danton wrote:
    > > > >
    > > > > On Tuesday, December 20, 2016 9:49 PM Michal Hocko wrote:
    > > > > >
    > > > > > @@ -1013,7 +1013,7 @@ bool out_of_memory(struct oom_control *oc)
    > > > > > * make sure exclude 0 mask - all other users should have at least
    > > > > > * ___GFP_DIRECT_RECLAIM to get here.
    > > > > > */
    > > > > > - if (oc->gfp_mask && !(oc->gfp_mask & (__GFP_FS|__GFP_NOFAIL)))
    > > > > > + if (oc->gfp_mask && !(oc->gfp_mask & __GFP_FS))
    > > > > > return true;
    > > > > >
    > > > > As to GFP_NOFS|__GFP_NOFAIL request, can we check gfp mask
    > > > > one bit after another?
    > > > >
    > > > > if (oc->gfp_mask) {
    > > > > if (!(oc->gfp_mask & __GFP_FS))
    > > > > return false;
    > > > >
    > > > > /* No service for request that can handle fail result itself */
    > > > > if (!(oc->gfp_mask & __GFP_NOFAIL))
    > > > > return false;
    > > > > }
    > > >
    > > > I really do not understand this request.
    > >
    > > It's a request of both NOFS and NOFAIL, and I think we can keep it from
    > > hitting oom killer by shuffling the current gfp checks.
    > > I hope it can make nit sense to your work.
    > >
    >
    > I still do not understand. The whole point we are doing the late
    > __GFP_FS check is explained in 3da88fb3bacf ("mm, oom: move GFP_NOFS
    > check to out_of_memory"). And the reason why I am _removing_
    > __GFP_NOFAIL is explained in the changelog of this patch.
    >
    > > > This patch is removing the __GFP_NOFAIL part...
    > >
    > > Yes, and I don't stick to handling NOFAIL requests inside oom.
    > >
    > > > Besides that why should they return false?
    > >
    > > It's feedback to page allocator that no kill is issued, and
    > > extra attention is needed.
    >
    > Be careful, the semantic of out_of_memory is different. Returning false
    > means that the oom killer has been disabled and so the allocation should
    > fail rather than loop for ever.
    >
    By returning false, I mean that oom killer is making no progress.
    And I prefer to give up looping if oom killer can't help.
    It's a change in the current semantic to fail the request and I have
    to test it isn't bad.

    thanks
    Hillf


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