Re: [PATCH RFC PKS/PMEM 48/58] drivers/md: Utilize new kmap_thread()

[Coly Li <colyli@xxxxxxx>]

On 2020/10/10 03:50, ira.weiny@xxxxxxxxx wrote:
> From: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> These kmap() calls are localized to a single thread.  To avoid the over
> head of global PKRS updates use the new kmap_thread() call.
> 

Hi Ira,

There were a number of options considered.

1) Attempt to change all the thread local kmap() calls to kmap_atomic()
2) Introduce a flags parameter to kmap() to indicate if the mapping
should be global or not
3) Change ~20-30 call sites to 'kmap_global()' to indicate that they
require a global mapping of the pages
4) Change ~209 call sites to 'kmap_thread()' to indicate that the
mapping is to be used within that thread of execution only


I copied the above information from patch 00/58 to this message. The
idea behind kmap_thread() is fine to me, but as you said the new api is
very easy to be missed in new code (even for me). I would like to be
supportive to option 2) introduce a flag to kmap(), then we won't forget
the new thread-localized kmap method, and people won't ask why a
_thread() function is called but no kthread created.

Thanks.


Coly Li



> Cc: Coly Li <colyli@xxxxxxx> (maintainer:BCACHE (BLOCK LAYER CACHE))
> Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx> (maintainer:BCACHE (BLOCK 
> LAYER CACHE))
> Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> ---
>  drivers/md/bcache/request.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
> index c7cadaafa947..a4571f6d09dd 100644
> --- a/drivers/md/bcache/request.c
> +++ b/drivers/md/bcache/request.c
> @@ -44,10 +44,10 @@ static void bio_csum(struct bio *bio, struct bkey *k)
>       uint64_t csum = 0;
>  
>       bio_for_each_segment(bv, bio, iter) {
> -             void *d = kmap(bv.bv_page) + bv.bv_offset;
> +             void *d = kmap_thread(bv.bv_page) + bv.bv_offset;
>  
>               csum = bch_crc64_update(csum, d, bv.bv_len);
> -             kunmap(bv.bv_page);
> +             kunmap_thread(bv.bv_page);
>       }
>  
>       k->ptr[KEY_PTRS(k)] = csum & (~0ULL >> 1);
>