Re: [PATCH 12/30] mm: memory reserve management

Peter Zijlstra
2008-08-12T08:12:18Z

On Tue, 2008-08-12 at 17:46 +1000, Neil Brown wrote:
> On Thursday July 24, a.p.zijlstra@chello.nl wrote:
> > Generic reserve management code.
> >
> > It provides methods to reserve and charge. Upon this, generic alloc/free style
> > reserve pools could be build, which could fully replace mempool_t
> > functionality.
>
> More comments on this patch .....
>
> > +void *___kmalloc_reserve(size_t size, gfp_t flags, int node, void *ip,
> > + struct mem_reserve *res, int *emerg);
> > +
> > +static inline
> > +void *__kmalloc_reserve(size_t size, gfp_t flags, int node, void *ip,
> > + struct mem_reserve *res, int *emerg)
> > +{
> > + void *obj;
> > +
> > + obj = __kmalloc_node_track_caller(size,
> > + flags | __GFP_NOMEMALLOC | __GFP_NOWARN, node, ip);
> > + if (!obj)
> > + obj = ___kmalloc_reserve(size, flags, node, ip, res, emerg);
> > +
> > + return obj;
> > +}
> > +
> > +#define kmalloc_reserve(size, gfp, node, res, emerg) \
> > + __kmalloc_reserve(size, gfp, node, \
> > + __builtin_return_address(0), res, emerg)
> > +
> ......
> > +/*
> > + * alloc wrappers
> > + */
> > +
> > +void *___kmalloc_reserve(size_t size, gfp_t flags, int node, void *ip,
> > + struct mem_reserve *res, int *emerg)
> > +{
> > + void *obj;
> > + gfp_t gfp;
> > +
> > + gfp = flags | __GFP_NOMEMALLOC | __GFP_NOWARN;
> > + obj = __kmalloc_node_track_caller(size, gfp, node, ip);
> > +
> > + if (obj || !(gfp_to_alloc_flags(flags) & ALLOC_NO_WATERMARKS))
> > + goto out;
> > +
> > + if (res && !mem_reserve_kmalloc_charge(res, size)) {
> > + if (!(flags & __GFP_WAIT))
> > + goto out;
> > +
> > + wait_event(res->waitqueue,
> > + mem_reserve_kmalloc_charge(res, size));
> > +
> > + obj = __kmalloc_node_track_caller(size, gfp, node, ip);
> > + if (obj) {
> > + mem_reserve_kmalloc_charge(res, -size);
> > + goto out;
> > + }
> > + }
> > +
> > + obj = __kmalloc_node_track_caller(size, flags, node, ip);
> > + WARN_ON(!obj);
> > + if (emerg)
> > + *emerg |= 1;
> > +
> > +out:
> > + return obj;
> > +}
>
> Two comments to be precise.
>
> 1/ __kmalloc_reserve attempts a __GFP_NOMEMALLOC allocation, and then
> if that fails, ___kmalloc_reserve immediately tries again.
> Is that pointless? Should the second one be removed?

Pretty pointless yes, except that it made ___kmalloc_reserve a nicer
function to read, and as its an utter slow path I couldn't be arsed to
optimize :-)

> 2/ mem_reserve_kmalloc_charge appears to assume that the 'mem_reserve'
> has been 'connected' and so is active.

Hmm, that would be __mem_reserve_charge() then, because the callers
don't do much.

> While callers probably only set GFP_MEMALLOC in cases where the
> mem_reserve is connected, ALLOC_NO_WATERMARKS could get via
> PF_MEMALLOC so we could end up calling mem_reserve_kmalloc_charge
> when the mem_reserve is not connected.

Right..

> That seems to be 'odd' at least.
> It might even be 'wrong' as mem_reserve_connect doesn't add the
> usage of the child to the parent - only the ->pages and ->limit.
>
> What is your position on this? Mine is "still slightly confused".

Uhmm,. good point. Let me ponder this while I go for breakfast ;-)