Re: [PATCH] mm/slub: Reduce memory consumption in extreme scenarios

[Mike Rapoport]

On Mon, Mar 20, 2023 at 09:05:57AM +0100, Vlastimil Babka wrote:
> On 3/19/23 08:22, chenjun (AM) wrote:
> > 在 2023/3/17 20:06, Vlastimil Babka 写道:
> >> On 3/17/23 12:32, chenjun (AM) wrote:
> >>> 在 2023/3/14 22:41, Vlastimil Babka 写道:
> >>>>>    	pc.flags = gfpflags;
> >>>>> +
> >>>>> +	/*
> >>>>> +	 * when (node != NUMA_NO_NODE) && (gfpflags & __GFP_THISNODE)
> >>>>> +	 * 1) try to get a partial slab from target node with __GFP_THISNODE.
> >>>>> +	 * 2) if 1) failed, try to allocate a new slab from target node with
> >>>>> +	 *    __GFP_THISNODE.
> >>>>> +	 * 3) if 2) failed, retry 1) and 2) without __GFP_THISNODE constraint.
> >>>>> +	 */
> >>>>> +	if (node != NUMA_NO_NODE && !(gfpflags & __GFP_THISNODE) && try_thisnode)
> >>>>> +			pc.flags |= __GFP_THISNODE;
> >>>>
> >>>> Hmm I'm thinking we should also perhaps remove direct reclaim possibilities
> >>>> from the attempt 2). In your qemu test it should make no difference, as it
> >>>> fills everything with kernel memory that is not reclaimable. But in practice
> >>>> the target node might be filled with user memory, and I think it's better to
> >>>> quickly allocate on a different node than spend time in direct reclaim. So
> >>>> the following should work I think?
> >>>>
> >>>> pc.flags = GFP_NOWAIT | __GFP_NOWARN |__GFP_THISNODE
> >>>>
> >>>
> >>> Hmm, Should it be that:
> >>>
> >>> pc.flags |= GFP_NOWAIT | __GFP_NOWARN |__GFP_THISNODE
> >> 
> >> No, we need to ignore the other reclaim-related flags that the caller
> >> passed, or it wouldn't work as intended.
> >> The danger is that we ignore some flag that would be necessary to pass, but
> >> I don't think there's any?
> >> 
> >> 
> > 
> > If we ignore __GFP_ZERO passed by kzalloc， kzalloc will not work.
> > Could we just unmask __GFP_RECLAIMABLE | __GFP_RECLAIM?
> > 
> > pc.flags &= ~(__GFP_RECLAIMABLE | __GFP_RECLAIM)
> > pc.flags |= __GFP_THISNODE
> 
> __GFP_RECLAIMABLE would be wrong, but also ignored as new_slab() does:
> 	flags & (GFP_RECLAIM_MASK | GFP_CONSTRAINT_MASK)
> 
> which would filter out __GFP_ZERO as well. That's not a problem as kzalloc()
> will zero out the individual allocated objects, so it doesn't matter if we
> don't zero out the whole slab page.
> 
> But I wonder, if we're not past due time for a helper e.g.
> gfp_opportunistic(flags) that would turn any allocation flags to a
> GFP_NOWAIT while keeping the rest of relevant flags intact, and thus there
> would be one canonical way to do it - I'm sure there's a number of places
> with their own variants now?
> With such helper we'd just add __GFP_THISNODE to the result here as that's
> specific to this particular opportunistic allocation.

I like the idea, but maybe gfp_no_reclaim() would be clearer?

-- 
Sincerely yours,
Mike.