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

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Message-ID: <322136ff-95f6-49f6-9126-05845f25b4e8@kernel.dk>
Date: Tue, 20 Jan 2026 08:01:52 -0700
From: Jens Axboe <axboe@...nel.dk>
To: Coly Li <colyli@...as.com>, Stephen Zhang <starzhangzsd@...il.com>
Cc: Kent Overstreet <kent.overstreet@...ux.dev>,
 Sasha Levin <sashal@...nel.org>, Christoph Hellwig <hch@...radead.org>,
 linux-bcache@...r.kernel.org,
 Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
 zhangshida <zhangshida@...inos.cn>
Subject: Re: Fwd: [PATCH v2] bcache: use bio cloning for detached device
 requests

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).

-- 
Jens Axboe