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Wed, 22 Mar 2023 04:06:16 -0700 (PDT) Received: from localhost ([2a00:23c5:dc8c:8701:1663:9a35:5a7b:1d76]) by smtp.gmail.com with ESMTPSA id m9-20020adffa09000000b002c70d97af78sm13647255wrr.85.2023.03.22.04.06.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 22 Mar 2023 04:06:15 -0700 (PDT) Date: Wed, 22 Mar 2023 11:06:14 +0000 From: Lorenzo Stoakes To: Baoquan He Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Andrew Morton , Uladzislau Rezki , Matthew Wilcox , David Hildenbrand , Liu Shixin , Jiri Olsa , Jens Axboe , Alexander Viro Subject: Re: [PATCH v4 3/4] iov_iter: add copy_page_to_iter_atomic() Message-ID: References: <31482908634cbb68adafedb65f0b21888c194a1b.1679431886.git.lstoakes@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Queue-Id: 0D9E380019 X-Rspamd-Server: rspam01 X-Stat-Signature: myog9rinicpfpd1wu39eigjrjhiy71xd X-HE-Tag: 1679483177-226882 X-HE-Meta: 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 0ZVnpz4H MiKzPBWIB1U+C3lhOqikogrncw8qFpuRd4GpdnkLTn8ydtv29ojeu6zE0uHateA3v5OuaL8qiWl0V0KEJWRIXYNvDo3AFqwWbZiRasFCf2/ibKda/GBri9Zz2MnQU6bGAUf21qUolAayCMp6HjauNoE505K0G31+8CpA7n6tMqeQO7p6ckVuibJ/Xo0NVUK7BavEf9cCO7KrcDHSIDGX7SmMTLHTLaJLSVndFCn1xjNvy8IMaWrGCUxWJZzQYHQaMuJMgNnijljHlFZmVmv5PTwobTPnMqj3YIOM4RGtykF+qvsfBXlFGaSO8Ubfh+yz+JmGOHbvat3Ldhzye6uBXfRUfHzSlb1lzOkg1659BCH2h5OMCUEJeF1oi/vtKQm90eT9jw4dj2Ho5KD7Wl3fOJmU4/NUfwsPkoOoVDimplrnlwztGtytn6ug+2g20eJBw5axo96lNXSS4rYDGhrujhPtYNg== 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 Wed, Mar 22, 2023 at 10:32:47AM +0000, Lorenzo Stoakes wrote: > > I am a little confused about the name of this new function. In its > > conterpart, copy_page_from_iter_atomic(), kmap_atomic()/kunmpa_atomic() > > are used. With them, if CONFIG_HIGHMEM=n, it's like below: > > The reason for this is that:- > > 1. kmap_atomic() explicitly states that it is now deprecated and must no longer > be used, and kmap_local_page() should be used instead:- > > * kmap_atomic - Atomically map a page for temporary usage - Deprecated! > > * Do not use in new code. Use kmap_local_page() instead. > > 2. kmap_local_page() explicitly states that it can be used in any context:- > > * Can be invoked from any context, including interrupts. > > I wanted follow this advice as strictly as I could, hence the change. However, > we do need preemption/pagefaults explicitly disabled in this context (we are > happy to fail if the faulted in pages are unmapped in meantime), and I didn't > check the internals to make sure. > > So I think for safety it is better to use k[un]map_atomic() here, I'll respin > and put that back in, good catch! > Actually, given we have preemption disabled due to the held spinlock, I think it'd be better to add a copy_page_to_iter_nofault() that uses copy_to_user_nofault() which will disable pagefaults thus have exactly the equivalent behaviour, more explicitly and without the use of a deprecated function. Thanks for raising this!! > > > > static inline void *kmap_atomic(struct page *page) > > { > > if (IS_ENABLED(CONFIG_PREEMPT_RT)) > > migrate_disable(); > > else > > preempt_disable(); > > pagefault_disable(); > > return page_address(page); > > } > > > > But kmap_local_page() is only having page_address(), the code block > > between kmap_local_page() and kunmap_local() is also atomic, it's a > > little messy in my mind. > > > > static inline void *kmap_local_page(struct page *page) > > { > > return page_address(page); > > } > > > > > + char *p = kaddr + offset; > > > + size_t copied = 0; > > > + > > > + if (!page_copy_sane(page, offset, bytes) || > > > + WARN_ON_ONCE(i->data_source)) > > > + goto out; > > > + > > > + if (unlikely(iov_iter_is_pipe(i))) { > > > + copied = copy_page_to_iter_pipe(page, offset, bytes, i); > > > + goto out; > > > + } > > > + > > > + iterate_and_advance(i, bytes, base, len, off, > > > + copyout(base, p + off, len), > > > + memcpy(base, p + off, len) > > > + ) > > > + copied = bytes; > > > + > > > +out: > > > + kunmap_local(kaddr); > > > + return copied; > > > +} > > > +EXPORT_SYMBOL(copy_page_to_iter_atomic); > > > + > > > static void pipe_advance(struct iov_iter *i, size_t size) > > > { > > > struct pipe_inode_info *pipe = i->pipe; > > > -- > > > 2.39.2 > > > > >