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SubjectRe: [PATCH RFC PKS/PMEM 48/58] drivers/md: Utilize new kmap_thread()
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On 2020/10/10 03:50, ira.weiny@intel.com wrote:
> From: Ira Weiny <ira.weiny@intel.com>
>
> 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@suse.de> (maintainer:BCACHE (BLOCK LAYER CACHE))
> Cc: Kent Overstreet <kent.overstreet@gmail.com> (maintainer:BCACHE (BLOCK LAYER CACHE))
> Signed-off-by: Ira Weiny <ira.weiny@intel.com>
> ---
> 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);
>

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