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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, 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 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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