linux-kernel - Re: blk: improve order of bio handling in generic_make

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Message-ID: <87shmngey7.fsf@notabene.neil.brown.name>
Date:   Thu, 09 Mar 2017 17:08:16 +1100
From:   NeilBrown <neilb@...e.com>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Mike Snitzer <snitzer@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Jack Wang <jinpu.wang@...fitbricks.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Lars Ellenberg <lars.ellenberg@...bit.com>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Pavel Machek <pavel@....cz>
Subject: Re: blk: improve order of bio handling in generic_make_request()

On Wed, Mar 08 2017, Mikulas Patocka wrote:

> On Wed, 8 Mar 2017, NeilBrown wrote:
>> 
>> I don't think this will fix the DM snapshot deadlock by itself.
>> Rather, it make it possible for some internal changes to DM to fix it.
>> The DM change might be something vaguely like:
>> 
>> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
>> index 3086da5664f3..06ee0960e415 100644
>> --- a/drivers/md/dm.c
>> +++ b/drivers/md/dm.c
>> @@ -1216,6 +1216,14 @@ static int __split_and_process_non_flush(struct clone_info *ci)
>> 
>>  	len = min_t(sector_t, max_io_len(ci->sector, ti), ci->sector_count);
>> 
>> +	if (len < ci->sector_count) {
>> +		struct bio *split = bio_split(bio, len, GFP_NOIO, fs_bio_set);
>
> fs_bio_set is a shared bio set, so it is prone to deadlocks. For this 
> change, we would need two bio sets per dm device, one for the split bio 
> and one for the outgoing bio. (this also means having one more kernel 
> thread per dm device)

Yes, two local bio_sets would be best.
But we don't really need those extra kernel threads.  I'll start working
on patches to make them optional, and then to start removing them.

Thanks,
NeilBrown

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