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Miller" , sparclinux@vger.kernel.org, Chris Zankel , Max Filippov , linux-xtensa@linux-xtensa.org Subject: Re: [patch V2 00/18] mm/highmem: Preemptible variant of kmap_atomic & friends Message-ID: <20201030130627.GI27442@casper.infradead.org> References: <20201029221806.189523375@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201029221806.189523375@linutronix.de> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Oct 29, 2020 at 11:18:06PM +0100, Thomas Gleixner wrote: > This series provides kmap_local.* iomap_local variants which only disable > migration to keep the virtual mapping address stable accross preemption, > but do neither disable pagefaults nor preemption. The new functions can be > used in any context, but if used in atomic context the caller has to take > care of eventually disabling pagefaults. Could I ask for a CONFIG_KMAP_DEBUG which aliases all the kmap variants to vmap()? I think we currently have a problem in iov_iter on HIGHMEM configs: copy_page_to_iter() calls page_copy_sane() which checks: head = compound_head(page); if (likely(n <= v && v <= page_size(head))) return true; but then: void *kaddr = kmap_atomic(page); size_t wanted = copy_to_iter(kaddr + offset, bytes, i); kunmap_atomic(kaddr); so if offset to offset+bytes is larger than PAGE_SIZE, this is going to work for lowmem pages and fail miserably for highmem pages. I suggest vmap() because vmap has a PAGE_SIZE gap between each allocation. Alternatively if we could have a kmap_atomic_compound(), that would be awesome, but probably not realistic to implement. I've more or less resigned myself to having to map things one page at a time.