Re: reusing bios

[Arne Jansen <lists@die-jansens.de>]

Jens Axboe wrote:
> On 2011-03-12 14:33, Arne Jansen wrote:
>> Jens Axboe wrote:
>>> On 2011-03-12 10:29, Arne Jansen wrote:
>>>> Hi,
>>>>
>>>> I'm trying to re-use struct bio after completion, including the
>>>> allocated pages. Normally I re-initialize the fields bi_sector,
>>>> bi_size, bi_next, bi_flags, bi_comp_cpu and bi_bdev.
>>>> This works perfectly well, as long as no media errors are encountered.
>>>> After a media error, all subsequent reads with this bio fail. Are
>>>> there any more fields that need to get re-initialized? Or, better,
>>>> is there a function to reset the bio?
>>> bio_init()? Sounds like you are not setting BIO_UPTODATE when resetting
>>> it.
>>>
>> bio_init does way too much, as it discards the io_vec, refcnt, destructor,
>> private etc.
>> I set the flags to 1 << BIO_UPTODATE, but that proved to be fatal, too, as
>> it clobbers the POOL_FLAGS, which leads to an oops when the last reference
>> drops. These bio-beasts are just not made to be reused. One needs way too
>> much internal knowledge to reinitialize them properly.
>> Therefore, I'd really like to have a call like bio_reinit, which keeps the
>> io_vec and all the owners private information, but resets the parts that
>> get used by the stack like bi_next. Would that make sense?
> 
> The thing is, you can't really reuse without violating the allocator
> principle of finite life times. Otherwise you risk hanging the system.
> If you want to reuse, the bio should come out of your own pool or
> through the bio_kmalloc() interface, not from bio_alloc() with the
> default pool.
> 
> So there is no reinit, as there hasn't been a good use case for that.
> The fact that flags has pool information in the top bits is a sure sign
> you are violating that.
> 

ok, assuming I allocate them with bio_kmalloc, the problem of reinit would
remain. Now that I own them, I can't reuse them.
This particular use case here is a scrub for btrfs. I have a limited set of
bios and pages, that I reuse as long as the scrub runs. Of course I could
just free up and reallocate everything after each read, but as I know how
much work there is ahead, I can just as well keep them.