linux-ext4 - Re: [PATCH RFC PKS/PMEM 22/58] fs/f2fs: Utilize new kmap

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Date:   Mon, 12 Oct 2020 09:19:46 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
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Subject: Re: [PATCH RFC PKS/PMEM 22/58] fs/f2fs: Utilize new kmap_thread()

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()?

> And since I don't know the call site details if there are kmap_thread() calls
> which are better off as kmap_atomic() calls I think it is worth converting
> them.  But I made the assumption that kmap users would already be calling
> kmap_atomic() if they could (because it is more efficient).

Not necessarily.  In cases where either one is correct, people might not have
put much thought into which of kmap() and kmap_atomic() they are using.

- Eric