Re: [PATCH] cpuset,mm: fix allocating page cache/slab object on the unallowed node when memory spread is set

Andrew Morton
2008-12-30T22:28:53Z

On Fri, 26 Dec 2008 14:37:07 +0800
Miao Xie <miaox@cn.fujitsu.com> wrote:

> The task still allocated the page caches on old node after modifying its
> cpuset's mems when 'memory_spread_page' was set, it is caused by the old
> mem_allowed_list of the task. Slab has the same problem.

ok...

> diff --git a/mm/filemap.c b/mm/filemap.c
> index f3e5f89..d978983 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -517,6 +517,9 @@ int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
> #ifdef CONFIG_NUMA
> struct page *__page_cache_alloc(gfp_t gfp)
> {
> + if ((gfp & __GFP_WAIT) && !in_interrupt())
> + cpuset_update_task_memory_state();
> +
> if (cpuset_do_page_mem_spread()) {
> int n = cpuset_mem_spread_node();
> return alloc_pages_node(n, gfp, 0);
> diff --git a/mm/slab.c b/mm/slab.c
> index 0918751..3b6e3d7 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -3460,6 +3460,9 @@ __cache_alloc(struct kmem_cache *cachep, gfp_t flags, void *caller)
> if (should_failslab(cachep, flags))
> return NULL;
>
> + if ((flags & __GFP_WAIT) && !in_interrupt())
> + cpuset_update_task_memory_state();
> +
> cache_alloc_debugcheck_before(cachep, flags);
> local_irq_save(save_flags);
> objp = __do_cache_alloc(cachep, flags);

Problems.

a) There's no need to test in_interrupt(). Any caller who passed us
__GFP_WAIT from interrupt context is horridly buggy and needs to be
fixed.

b) Even if the caller _did_ set __GFP_WAIT, there's no guarantee
that we're deadlock safe here. Does anyone ever do a __GFP_WAIT
allocation while holding callback_mutex? If so, it'll deadlock.

c) These are two of the kernel's hottest code paths. We really
really really really don't want to be polling for some dopey
userspace admin change on each call to __cache_alloc()!

d) How does slub handle this problem?