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

[Mikulas Patocka <mpatocka@redhat.com>]

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.

"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