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

Neil Brown
2008-08-12T07:46:39Z

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?

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

NeilBrown