Received-SPF: pass (google.com: domain of linux-nfs-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
IronPort-SDR: sHEBSwDrhTzQU2S1NEllE7XlTU+xF+PhbrOIMhK+ymP1xuGREJ/BLRLAOafpJKPLRy0iKRj28m
 +ZZbJv87UD9w==
IronPort-SDR: fkO3cV1AstS50IlWJFeIR7IMIWTUvxOVu0RxBH9tBAb2aFjzmjvlioAsa4gAQcEu3X0zxNtHVX
 KqidbmrXoAPg==
Date:   Mon, 12 Oct 2020 12:53:54 -0700
From:   Ira Weiny <ira.weiny@intel.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Eric Biggers <ebiggers@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, linux-aio@kvack.org,
        linux-efi@vger.kernel.org, kvm@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mmc@vger.kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        dri-devel@lists.freedesktop.org, linux-mm@kvack.org,
        target-devel@vger.kernel.org, linux-mtd@lists.infradead.org,
        linux-kselftest@vger.kernel.org, samba-technical@lists.samba.org,
        ceph-devel@vger.kernel.org, drbd-dev@lists.linbit.com,
        devel@driverdev.osuosl.org, linux-cifs@vger.kernel.org,
        linux-nilfs@vger.kernel.org, linux-scsi@vger.kernel.org,
        linux-nvdimm@lists.01.org, linux-rdma@vger.kernel.org,
        x86@kernel.org, amd-gfx@lists.freedesktop.org,
        linux-afs@lists.infradead.org, cluster-devel@redhat.com,
        linux-cachefs@redhat.com, intel-wired-lan@lists.osuosl.org,
        xen-devel@lists.xenproject.org, linux-ext4@vger.kernel.org,
        Fenghua Yu <fenghua.yu@intel.com>, ecryptfs@vger.kernel.org,
        linux-um@lists.infradead.org, intel-gfx@lists.freedesktop.org,
        linux-erofs@lists.ozlabs.org, reiserfs-devel@vger.kernel.org,
        linux-block@vger.kernel.org, linux-bcache@vger.kernel.org,
        Jaegeuk Kim <jaegeuk@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        io-uring@vger.kernel.org, linux-nfs@vger.kernel.org,
        linux-ntfs-dev@lists.sourceforge.net, netdev@vger.kernel.org,
        kexec@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net,
        linux-fsdevel@vger.kernel.org, bpf@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH RFC PKS/PMEM 22/58] fs/f2fs: Utilize new kmap_thread()
Message-ID: <20201012195354.GC2046448@iweiny-DESK2.sc.intel.com>
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>
 <20201012164438.GA20115@casper.infradead.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20201012164438.GA20115@casper.infradead.org>
User-Agent: Mutt/1.11.1 (2018-12-01)
Precedence: bulk

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.