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Wed, 21 Jan 2026 06:50:38 -0800 (PST) Message-ID: <9a928d95-f027-4f0d-b34b-ca028ad36046@kernel.dk> Date: Wed, 21 Jan 2026 07:50:37 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Fwd: [PATCH v2] bcache: use bio cloning for detached device requests To: Stephen Zhang , Coly Li Cc: Kent Overstreet , Sasha Levin , Christoph Hellwig , linux-bcache@vger.kernel.org, Linux Kernel Mailing List , zhangshida References: <20260120023535.9109-1-zhangshida@kylinos.cn> <322136ff-95f6-49f6-9126-05845f25b4e8@kernel.dk> Content-Language: en-US From: Jens Axboe In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 1/21/26 4:55 AM, Stephen Zhang wrote: > Coly Li 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