Re: [PATCH 2/3] virtual block device driver (ramzswap)

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Thu, 24 Sep 2009 22:24:50 +0530
Nitin Gupta <ngupta@vflare.org> wrote:

> 
> On 09/24/2009 10:41 AM, KAMEZAWA Hiroyuki wrote:
> > On Tue, 22 Sep 2009 10:26:53 +0530
> > Nitin Gupta <ngupta@vflare.org> wrote:
> > 
> > <snip>
> >> +	if (unlikely(clen > max_zpage_size)) {
> >> +		if (rzs->backing_swap) {
> >> +			mutex_unlock(&rzs->lock);
> >> +			fwd_write_request = 1;
> >> +			goto out;
> >> +		}
> >> +
> >> +		clen = PAGE_SIZE;
> >> +		page_store = alloc_page(GFP_NOIO | __GFP_HIGHMEM);
> > Here, and...
> > 
> >> +		if (unlikely(!page_store)) {
> >> +			mutex_unlock(&rzs->lock);
> >> +			pr_info("Error allocating memory for incompressible "
> >> +				"page: %u\n", index);
> >> +			stat_inc(rzs->stats.failed_writes);
> >> +			goto out;
> >> +		}
> >> +
> >> +		offset = 0;
> >> +		rzs_set_flag(rzs, index, RZS_UNCOMPRESSED);
> >> +		stat_inc(rzs->stats.pages_expand);
> >> +		rzs->table[index].page = page_store;
> >> +		src = kmap_atomic(page, KM_USER0);
> >> +		goto memstore;
> >> +	}
> >> +
> >> +	if (xv_malloc(rzs->mem_pool, clen + sizeof(*zheader),
> >> +			&rzs->table[index].page, &offset,
> >> +			GFP_NOIO | __GFP_HIGHMEM)) {
> > 
> > Here.
> >     
> > Do we need to wait until here for detecting page-allocation-failure ?
> > Detecting it here means -EIO for end_swap_bio_write()....unhappy
> > ALERT messages etc..
> > 
> > Can't we add a hook to get_swap_page() for preparing this ("do we have
> > enough pool?") and use only GFP_ATOMIC throughout codes ?
> > (memory pool for this swap should be big to some extent.)
> >
> 
> Yes, we do need to wait until this step for detecting alloc failure since
> we don't really know when pool grow will (almost) surely wail.
> What we can probably do is, hook into OOM notify chain (oom_notify_list)
> and whenever we get this callback, we can start sending pages directly
> to backing swap and do not even attempt to do any allocation.
> 
> 
Hmm...then, I never see -EIO ?

>  
> >>From my user support experience for heavy swap customers,  extra memory allocation for swapping out is just bad...in many cases.
> > (*) I know GFP_IO works well to some extent.
> > 
> 
> We cannot use GFP_IO here as it can cause a deadlock:
> ramzswap alloc() --> not enough memory, try to reclaim some --> swap out ...
> ... some pages to ramzswap --> ramzswap alloc()
> 
Ah, sorry. just my mistake. I wanted to write GFP_NOIO.

Thanks,
-Kame



> Thanks,
> Nitin
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