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Date:   Mon, 12 Oct 2020 17:44:38 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Eric Biggers <ebiggers@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        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 Mon, Oct 12, 2020 at 09:28:29AM -0700, Dave Hansen wrote:
> 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.

But kmap_atomic() disables preemption, so the _ideal_ interface would map
it only locally, then on preemption make it global.  I don't even know
if that _can_ be done.  But this email makes it seem like kmap_atomic()
has no downsides.

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