Re: NILFS2 get stuck after bio_alloc() fail

[Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>]

Hi,
On Sun, 14 Jun 2009 10:00:06 +0300, Pekka Enberg <penberg@cs.helsinki.fi> wrote:
> On Sun, Jun 14, 2009 at 9:30 AM, Ryusuke
> Konishi<konishi.ryusuke@lab.ntt.co.jp> wrote:
>> The original GFP flag was GFP_NOIO, but replaced to GFP_NOWAIT at a
>> preliminary release in February 2008.  It was because a user
>> experienced system memory shortage by the bio_alloc() call.
>>
>> Even though nilfs_alloc_seg_bio() repeatedly calls bio_alloc()
>> reducing the number of bio vectors in case of failure, this fallback
>> did not work well.
>>
>> I'm in two minds whether I should change it back to GFP_NOIO.
>> Or should I switch the gfp as follows?
> 
> As far as I can tell, the only difference with GFP_NOIO and GFP_NOWAIT
> here is that the former will trigger the mempool_alloc() page reclaim
> path. But I am not sure I understand why switching to GFP_NOWAIT
> helped with memory shortage. What exactly was the problem there?
> 
>                                  Pekka

I will confirm the details later.  I cannot dig into the record due to
planned outage at office now.  The version I replaced GFP_NOIO with
GFP_NOWAIT is considerably old.  A true problem might have been in
other places somewhere.

For reference, the retry loop in question is as follows.  It is to
allocate bio for log writing.

        bio = bio_alloc(GFP_NOWAIT, nr_vecs);
        if (bio == NULL) {
                while (!bio && (nr_vecs >>= 1))
                       bio = bio_alloc(GFP_NOWAIT, nr_vecs);
        }

where nr_vecs is less than or equal to bio_get_nr_vecs(bdev).


Thanks,
Ryusuke Konishi