Re: NILFS2 get stuck after bio_alloc() fail

From: Ryusuke Konishi
Date: Sun Jun 14 2009 - 08:34:57 EST

On Sun, 14 Jun 2009 10:00:06 +0300, Pekka Enberg <penberg@xxxxxxxxxxxxxx> wrote:
> On Sun, Jun 14, 2009 at 9:30 AM, Ryusuke
> Konishi<konishi.ryusuke@xxxxxxxxxxxxx> 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).

Ryusuke Konishi
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