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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>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, linux-aio@...ck.org,
        linux-efi@...r.kernel.org, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mmc@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        dri-devel@...ts.freedesktop.org, linux-mm@...ck.org,
        target-devel@...r.kernel.org, linux-mtd@...ts.infradead.org,
        linux-kselftest@...r.kernel.org, samba-technical@...ts.samba.org,
        ceph-devel@...r.kernel.org, drbd-dev@...ts.linbit.com,
        devel@...verdev.osuosl.org, linux-cifs@...r.kernel.org,
        linux-nilfs@...r.kernel.org, linux-scsi@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-rdma@...r.kernel.org,
        x86@...nel.org, amd-gfx@...ts.freedesktop.org,
        linux-afs@...ts.infradead.org, cluster-devel@...hat.com,
        linux-cachefs@...hat.com, intel-wired-lan@...ts.osuosl.org,
        xen-devel@...ts.xenproject.org, linux-ext4@...r.kernel.org,
        Fenghua Yu <fenghua.yu@...el.com>, ecryptfs@...r.kernel.org,
        linux-um@...ts.infradead.org, intel-gfx@...ts.freedesktop.org,
        linux-erofs@...ts.ozlabs.org, reiserfs-devel@...r.kernel.org,
        linux-block@...r.kernel.org, linux-bcache@...r.kernel.org,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        io-uring@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-ntfs-dev@...ts.sourceforge.net, netdev@...r.kernel.org,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, bpf@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-btrfs@...r.kernel.org
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

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