Re: Fwd: [PATCH v2] bcache: use bio cloning for detached device requests

[Jens Axboe <axboe@kernel.dk>]

On 1/21/26 4:55 AM, Stephen Zhang wrote:
> Coly Li <colyli@fnnas.com> 于2026年1月21日周三 09:34写道：
>>
>> On Tue, Jan 20, 2026 at 08:01:52AM +0800, Jens Axboe wrote:
>>> On 1/20/26 7:46 AM, Coly Li wrote:
>>>>> @@ -949,6 +950,11 @@ static int bcache_device_init(struct
>>>>> bcache_device *d, unsigned int block_size,
>>>>>                         BIOSET_NEED_BVECS|BIOSET_NEED_RESCUER))
>>>>>                 goto out_ida_remove;
>>>>>
>>>>> +       if (bioset_init(&d->bio_detach, 4,
>>>>                                     ^^^^^-> I feel 4 might be a bit small
>>>> here. bio_detached set is for normal IO when backing device is not
>>>> attached to a cache device. I would suggest to set the pool size to
>>>> 128 or 256.
>>>
>>> Absolutely not, 4 is more than plenty. The pool elements are only ever
>>> used if allocations fail, to guarantee forward progress. Setting aside
>>> 128 or 256 for that case is utterly wasteful, you only need a couple. 4
>>> is a good number, if anything it should be smaller (2).
>>
>> Hi Jens,
>>
>> Thanks for the information. Please correct me if I am wrong for the following
>> text,
>> - If the backing is a normal SSD raid0, the IOPS without attached cache device
>> might be more than thousands. In this case, I assume 128 or 256 might be more
>> tolerant.
>> - I see what ‘4’ means, just not sure/comfortable when memory pressure is high.
>> And reserving 128/256 will occupy around 0.5~1MB memory, I feel such extra
>> memory is acceptable in bcache use case.
>>
>> Don't get me wrong, I totally trust you. If '4' works well enough for high
>> memory pressure condition for detached bcache device, it is cool.
>>
>> Thanks in advance.
>>
>> Coly Li
> 
> Hi Jens, Coly,
> 
> Regarding the discussion on the bio_detached pool size: while 4 is
> sufficient to guarantee forward progress, some high-load environments
> may prefer a larger reserve to minimize allocation latency under
> extreme memory pressure.
> 
> To provide a middle ground, I propose adding a generic bioset_resize()
> interface to the block layer and exposing it through a new bcache
> sysfs attribute 'detached_pool_size'.
> 
> This allows us to keep the default value conservative (e.g., 4) to
> avoid unnecessary memory overhead, while giving users the flexibility
> to tune the emergency reserves based on their specific hardware and
> workload requirements.
> 
> The patch is attached below. Does this look like a reasonable
> compromise?

Guys, just stop. 4 is fine. Take a look at the default biosets and
what reserve amount they use. I'd recommend you do some testing and
check how often the reserves are _actually_ used, and once you do
that, then you'll see why this isn't necessary at all.

I thought the idea was to make some progress on getting this fixed.
Let's please do that and get it queued.

-- 
Jens Axboe