linux-kernel - Re: [PATCH v2 00/11] dm-pcache – persistent-memory cache for block devices

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Message-ID: <e50ef45e-4c1a-4874-8d5f-9ca86f9a532c@linux.dev>
Date: Wed, 9 Jul 2025 17:45:23 +0800
From: Dongsheng Yang <dongsheng.yang@...ux.dev>
To: Mikulas Patocka <mpatocka@...hat.com>
Cc: agk@...hat.com, snitzer@...nel.org, axboe@...nel.dk, hch@....de,
 dan.j.williams@...el.com, Jonathan.Cameron@...wei.com,
 linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-cxl@...r.kernel.org, nvdimm@...ts.linux.dev, dm-devel@...ts.linux.dev
Subject: Re: [PATCH v2 00/11] dm-pcache – persistent-memory cache for block devices


在 7/8/2025 4:16 AM, Mikulas Patocka 写道:
>
> On Mon, 7 Jul 2025, Dongsheng Yang wrote:
>
>> Hi Mikulas,
>> 	This is V2 for dm-pcache, please take a look.
>>
>> Code:
>>      https://github.com/DataTravelGuide/linux tags/pcache_v2
>>
>> Changelogs
>>
>> V2 from V1:
>> 	- introduce req_alloc() and req_init() in backing_dev.c, then we
>> 	  can do req_alloc() before holding spinlock and do req_init()
>> 	  in subtree_walk().
>> 	- introduce pre_alloc_key and pre_alloc_req in walk_ctx, that
>> 	  means we can pre-allocate cache_key or backing_dev_request
>> 	  before subtree walking.
>> 	- use mempool_alloc() with NOIO for the allocation of cache_key
>> 	  and backing_dev_req.
>> 	- some coding style changes from comments of Jonathan.
> Hi
>
> mempool_alloc with GFP_NOIO never fails - so you don't have to check the
> returned value for NULL and propagate the error upwards.


Hi Mikulas:

    I noticed that the implementation of mempool_alloc—it waits for 5 
seconds and retries when allocation fails.

With this in mind, I propose that we handle -ENOMEM inside defer_req() 
using a similar mechanism. something like this commit:


https://github.com/DataTravelGuide/linux/commit/e6fc2e5012b1fe2312ed7dd02d6fbc2d038962c0


Here are two key reasons why:

(1) If we manage -ENOMEM in defer_req(), we don’t need to modify every 
lower-level allocation to use mempool to avoid failures—for example,

cache_key, backing_req, and the kmem.bvecs you mentioned. More 
importantly, there’s no easy way to prevent allocation failure in some 
places—for instance, bio_init_clone() could still return -ENOMEM.

(2) If we use a mempool, it will block and wait indefinitely when memory 
is unavailable, preventing the process from exiting.

But with defer_req(), the user can still manually stop the pcache device 
using dmsetup remove, releasing some memory if user want.


What do you think?

Thanx

Dongsheng

>
> "backing_req->kmem.bvecs = kmalloc_array(n_vecs, sizeof(struct bio_vec),
> GFP_NOIO)" - this call may fail and you should handle the error gracefully
> (i.e. don't end the bio with an error). Would it be possible to trim the
> request to BACKING_DEV_REQ_INLINE_BVECS vectors and retry it?
> Alternativelly, you can create a mempool for the largest possible n_vecs
> and allocate from this mempool if kmalloc_array fails.
>
> I'm sending two patches for dm-pcache - the first patch adds the include
> file linux/bitfield.h - it is needed in my config. The second patch makes
> slab caches per-module rather than per-device, if you have them
> per-device, there are warnings about duplicate cache names.
>
>
> BTW. What kind of persistent memory do you use? (afaik Intel killed the
> Optane products and I don't know of any replacement)
>
> Some times ago I created a filesystem for persistent memory - see
> git://leontynka.twibright.com/nvfs.git - I'd be interested if you can test
> it on your persistent memory implementation.
>
> Mikulas
>