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Subject: Re: [PATCH RFC PKS/PMEM 48/58] drivers/md: Utilize new kmap_thread()
To:     ira.weiny@intel.com
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
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        Kent Overstreet <kent.overstreet@gmail.com>, x86@kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
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References: <20201009195033.3208459-1-ira.weiny@intel.com>
 <20201009195033.3208459-49-ira.weiny@intel.com>
From:   Coly Li <colyli@suse.de>
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Message-ID: <c802fbf4-f67a-b205-536d-9c71b440f9c8@suse.de>
Date:   Sat, 10 Oct 2020 10:20:34 +0800
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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);
>