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Date:   Mon, 12 Oct 2020 17:44:38 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Eric Biggers <ebiggers@kernel.org>,
        Ira Weiny <ira.weiny@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
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        linux-efi@vger.kernel.org, kvm@vger.kernel.org,
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Subject: Re: [PATCH RFC PKS/PMEM 22/58] fs/f2fs: Utilize new kmap_thread()
Message-ID: <20201012164438.GA20115@casper.infradead.org>
References: <20201009195033.3208459-1-ira.weiny@intel.com>
 <20201009195033.3208459-23-ira.weiny@intel.com>
 <20201009213434.GA839@sol.localdomain>
 <20201010003954.GW20115@casper.infradead.org>
 <20201010013036.GD1122@sol.localdomain>
 <20201012065635.GB2046448@iweiny-DESK2.sc.intel.com>
 <20201012161946.GA858@sol.localdomain>
 <5d621db9-23d4-e140-45eb-d7fca2093d2b@intel.com>
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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.