linux-kernel - Re: [PATCH RFC PKS/PMEM 22/58] fs/f2fs: Utilize new kmap

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Message-ID: <5d621db9-23d4-e140-45eb-d7fca2093d2b@intel.com>
Date:   Mon, 12 Oct 2020 09:28:29 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Eric Biggers <ebiggers@...nel.org>, Ira Weiny <ira.weiny@...el.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
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        Dave Hansen <dave.hansen@...ux.intel.com>,
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Subject: Re: [PATCH RFC PKS/PMEM 22/58] fs/f2fs: Utilize new kmap_thread()

On 10/12/20 9:19 AM, Eric Biggers wrote:
> On Sun, Oct 11, 2020 at 11:56:35PM -0700, Ira Weiny wrote:
>>> And I still don't really understand.  After this patchset, there is still code
>>> nearly identical to the above (doing a temporary mapping just for a memcpy) that
>>> would still be using kmap_atomic().
>> I don't understand.  You mean there would be other call sites calling:
>>
>> kmap_atomic()
>> memcpy()
>> kunmap_atomic()
> Yes, there are tons of places that do this.  Try 'git grep -A6 kmap_atomic'
> and look for memcpy().
> 
> Hence why I'm asking what will be the "recommended" way to do this...
> kunmap_thread() or kmap_atomic()?

kmap_atomic() is always preferred over kmap()/kmap_thread().
kmap_atomic() is _much_ more lightweight since its TLB invalidation is
always CPU-local and never broadcast.

So, basically, unless you *must* sleep while the mapping is in place,
kmap_atomic() is preferred.