Re: [PATCH v3 2/2] net/9p: add a per-client fcall kmem_cache

[piaojun]

Hi Dominique,

Could you help paste the test result of before-after-applied this patch in
comment? And please see my comments below.

On 2018/8/9 22:33, Dominique Martinet wrote:
> From: Dominique Martinet <dominique.martinet@xxxxxx>
> 
> Having a specific cache for the fcall allocations helps speed up
> allocations a bit, especially in case of non-"round" msizes.
> 
> The caches will automatically be merged if there are multiple caches
> of items with the same size so we do not need to try to share a cache
> between different clients of the same size.
> 
> Since the msize is negotiated with the server, only allocate the cache
> after that negotiation has happened - previous allocations or
> allocations of different sizes (e.g. zero-copy fcall) are made with
> kmalloc directly.
> 
> Signed-off-by: Dominique Martinet <dominique.martinet@xxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Greg Kurz <groug@xxxxxxxx>
> Cc: Jun Piao <piaojun@xxxxxxxxxx>
> ---
> v2:
>  - Add a pointer to the cache in p9_fcall to make sure a buffer
> allocated with kmalloc gets freed with kfree and vice-versa
> This could have been smaller with a bool but this spares having
> to look at the client so looked a bit cleaner, I'm expecting this
> patch will need a v3 one way or another so I went for the bolder
> approach - please say if you think a smaller item is better ; I *think*
> nothing relies on this being ordered the same way as the data on the
> wire (struct isn't packed anyway) so we can move id after tag and add
> another u8 to not have any overhead
> 
>  - added likely() to cache existence check in allocation, but nothing
> for msize check or free because of zc request being of different size
> 
> v3:
>  - defined the userdata region in the cache, as read or write calls can
> access the buffer directly (lead to warnings with HARDENED_USERCOPY=y)
> 
> 
> 
> I didn't get any comment on v2 for this patch, but I'm still not fully
> happy with this in all honesty.
> 
> Part of me believes we might be better off always allocating from the
> cache even for zero-copy headers, it's a waste of space but the buffers
> are there and being reused constantly for non-zc calls, and mixing
> kmallocs in could lead to these buffers being really freed and
> reallocated all the time instead of reusing the slab buffers
> continuously.
> 
> That would let me move the likely() for the fast path, with the only
> exception being the TVERSION initial call on mount, for which I still
> don't have any nice idea on how to handle, using a different size in
> p9_client_rpc or prepare_req if the type is TVERSION and I'm really not
> happy with that...
> 
> 
>  include/net/9p/9p.h     |  4 ++++
>  include/net/9p/client.h |  1 +
>  net/9p/client.c         | 40 +++++++++++++++++++++++++++++++++++-----
>  3 files changed, 40 insertions(+), 5 deletions(-)
> 
> diff --git a/include/net/9p/9p.h b/include/net/9p/9p.h
> index e23896116d9a..645266b74652 100644
> --- a/include/net/9p/9p.h
> +++ b/include/net/9p/9p.h
> @@ -336,6 +336,9 @@ enum p9_qid_t {
>  #define P9_NOFID	(u32)(~0)
>  #define P9_MAXWELEM	16
>  
> +/* Minimal header size: len + id + tag */

Here should be 'size + id + tag'.

> +#define P9_HDRSZ	7
> +
>  /* ample room for Twrite/Rread header */
>  #define P9_IOHDRSZ	24
>  
> @@ -558,6 +561,7 @@ struct p9_fcall {
>  	size_t offset;
>  	size_t capacity;
>  
> +	struct kmem_cache *cache;
>  	u8 *sdata;
>  };
>  
> diff --git a/include/net/9p/client.h b/include/net/9p/client.h
> index c2671d40bb6b..735f3979d559 100644
> --- a/include/net/9p/client.h
> +++ b/include/net/9p/client.h
> @@ -123,6 +123,7 @@ struct p9_client {
>  	struct p9_trans_module *trans_mod;
>  	enum p9_trans_status status;
>  	void *trans;
> +	struct kmem_cache *fcall_cache;
>  
>  	union {
>  		struct {
> diff --git a/net/9p/client.c b/net/9p/client.c
> index ed78751aee7c..6c57ab1294d7 100644
> --- a/net/9p/client.c
> +++ b/net/9p/client.c
> @@ -192,6 +192,9 @@ static int parse_opts(char *opts, struct p9_client *clnt)
>  				goto free_and_return;
>  			}
>  
> +			if (clnt->trans_mod) {
> +				pr_warn("Had trans %s\n", clnt->trans_mod->name);
> +			}
>  			v9fs_put_trans(clnt->trans_mod);
>  			clnt->trans_mod = v9fs_get_trans_by_name(s);
>  			if (clnt->trans_mod == NULL) {
> @@ -231,9 +234,16 @@ static int parse_opts(char *opts, struct p9_client *clnt)
>  	return ret;
>  }
>  
> -static int p9_fcall_init(struct p9_fcall *fc, int alloc_msize)
> +static int p9_fcall_init(struct p9_client *c, struct p9_fcall *fc,
> +			 int alloc_msize)
>  {
> -	fc->sdata = kmalloc(alloc_msize, GFP_NOFS);
> +	if (likely(c->fcall_cache) && alloc_msize == c->msize) {
> +		fc->sdata = kmem_cache_alloc(c->fcall_cache, GFP_NOFS);
> +		fc->cache = c->fcall_cache;
> +	} else {
> +		fc->sdata = kmalloc(alloc_msize, GFP_NOFS);
> +		fc->cache = NULL;
> +	}
>  	if (!fc->sdata)
>  		return -ENOMEM;
>  	fc->capacity = alloc_msize;
> @@ -242,7 +252,16 @@ static int p9_fcall_init(struct p9_fcall *fc, int alloc_msize)
>  
>  void p9_fcall_fini(struct p9_fcall *fc)
>  {
> -	kfree(fc->sdata);
> +	/* sdata can be NULL for interrupted requests in trans_rdma,
> +	 * and kmem_cache_free does not do NULL-check for us
> +	 */
> +	if (unlikely(!fc->sdata))
> +		return;
> +
> +	if (fc->cache)
> +		kmem_cache_free(fc->cache, fc->sdata);
> +	else
> +		kfree(fc->sdata);
>  }
>  EXPORT_SYMBOL(p9_fcall_fini);
>  
> @@ -267,9 +286,9 @@ p9_tag_alloc(struct p9_client *c, int8_t type, unsigned int max_size)
>  	if (!req)
>  		return NULL;
>  
> -	if (p9_fcall_init(&req->tc, alloc_msize))
> +	if (p9_fcall_init(c, &req->tc, alloc_msize))
>  		goto free_req;
> -	if (p9_fcall_init(&req->rc, alloc_msize))
> +	if (p9_fcall_init(c, &req->rc, alloc_msize))
>  		goto free;
>  
>  	p9pdu_reset(&req->tc);
> @@ -951,6 +970,7 @@ struct p9_client *p9_client_create(const char *dev_name, char *options)
>  
>  	clnt->trans_mod = NULL;
>  	clnt->trans = NULL;
> +	clnt->fcall_cache = NULL;
>  
>  	client_id = utsname()->nodename;
>  	memcpy(clnt->name, client_id, strlen(client_id) + 1);
> @@ -987,6 +1007,15 @@ struct p9_client *p9_client_create(const char *dev_name, char *options)
>  	if (err)
>  		goto close_trans;
>  
> +	/* P9_HDRSZ + 4 is the smallest packet header we can have that is
> +	 * followed by data accessed from userspace by read
> +	 */
> +	clnt->fcall_cache =
> +		kmem_cache_create_usercopy("9p-fcall-cache", clnt->msize,
> +					   0, 0, P9_HDRSZ + 4,
> +					   clnt->msize - (P9_HDRSZ + 4),
> +					   NULL);
> +
>  	return clnt;
>  
>  close_trans:
> @@ -1018,6 +1047,7 @@ void p9_client_destroy(struct p9_client *clnt)
>  
>  	p9_tag_cleanup(clnt);
>  
> +	kmem_cache_destroy(clnt->fcall_cache);

I'm afraid that we should check NULL for clnt->fcall_cache.

Thanks,
Jun

>  	kfree(clnt);
>  }
>  EXPORT_SYMBOL(p9_client_destroy);
>