[linux-mm-cc] Hi All

[IKEDA, Munehiro]

Hi all,

Nitin Gupta wrote:
>> 4) It decompresses page in context where we cannot sleep in general (like
>> in
>>> find_get_page() and friends). This is a serious drawback since we need
>> to
>>> allocate some temporary memory in decompression path also which we
>> cannot
>>> allow to sleep and thus has very high chances of failing - If
>> decompression
>>> path fails, we loose data and what not!
>> Why cannot we sleep?  The context is basically same as swap-in, which
>> may sleep the context.
>> In fact, my patch can make the context sleep and works well.
> 
> 
> I don't really remember what exactly failed when trying to sleep while
> decompressing. But you can easily find that out - in cc_readpage() do:
> 
> -    comp_page = alloc_page(GFP_ATOMIC);
> +    comp_page = alloc_page(GFP_KERNEL);
> 
> and you will see kernel panic/oops because you are sleeping in wrong
> context. Basically, we cannot determine in find_get_page() and friends as to
> whether we can sleep or not.

I guess it was caused by bit_spin_trylock() in handle_ccache_fault().
bit_spin_trylock() disables preempt so the context cannot sleep between 
lock and unlock.  As the result of it, alloc_pages() which is called 
from cc_readpage(), which is called from handle_ccache_fault(), cannot 
sleep.
If we want to sleep, we have to unlock before schedule and try to lock 
again after waken up.
Or, use bitlock like lock_page().
(I made a patch for it but not evaluated the performance hit)


> --------------
> Are you interested in the new implementation which is now under design?
> Probably, we can all meet up on IRC and discuss new plan?
> For all interested in this, please let me know time suitable for you (with
> time zone)
> 
> For me: 9:00 am (GMT +5:30) on 25/9 works.
> We can have 1/2-1 hour discussion.
> 
> If things go well, we can have weekly 15 mins sessions on IRC to discuss
> status and this will help us stay on track.

You mean 26/9?
I'm interested in it and want to discuss with you but unfortunately my 
network is behind damn firewall and I cannot use IRC now. Sigh.

> Cheers,
> Nitin


Best regards,
IKEDA, Munehiro