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Date:   Mon, 12 Oct 2020 12:53:54 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Eric Biggers <ebiggers@...nel.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,
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        x86@...nel.org, amd-gfx@...ts.freedesktop.org,
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        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,
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        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 05:44:38PM +0100, Matthew Wilcox wrote:
> 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.

And that is IIUC what Thomas was trying to solve.

Also, Linus brought up that kmap_atomic() has quirks in nesting.[1]

>From what I can see all of these discussions support the need to have something
between kmap() and kmap_atomic().

However, the reason behind converting call sites to kmap_thread() are different
between Thomas' patch set and mine.  Both require more kmap granularity.
However, they do so with different reasons and underlying implementations but
with the _same_ resulting semantics; a thread local mapping which is
preemptable.[2]  Therefore they each focus on changing different call sites.

While this patch set is huge I think it serves a valuable purpose to identify a
large number of call sites which are candidates for this new semantic.

Ira

[1] https://lore.kernel.org/lkml/CAHk-=wgbmwsTOKs23Z=71EBTrULoeaH2U3TNqT2atHEWvkBKdw@mail.gmail.com/
[2] It is important to note these implementations are not incompatible with
each other.  So I don't see yet another 'kmap_something()' being required.

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