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Fri, 1 Mar 2024 10:13:28 +0800 Message-ID: <8d49ad72-4d51-27b9-1c0e-0948942f8027@huawei.com> Date: Fri, 1 Mar 2024 10:13:28 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.8.0 Subject: Re: [bug report] dead loop in generic_perform_write() //Re: [PATCH v7 07/12] iov_iter: Convert iterate*() to inline funcs To: Linus Torvalds , Al Viro CC: David Howells , Jens Axboe , Christoph Hellwig , Christian Brauner , David Laight , Matthew Wilcox , Jeff Layton , , , , , , Kefeng Wang References: <20230925120309.1731676-1-dhowells@redhat.com> <20230925120309.1731676-8-dhowells@redhat.com> <4e80924d-9c85-f13a-722a-6a5d2b1c225a@huawei.com> From: Tong Tiangen In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.179.234] X-ClientProxiedBy: dggems705-chm.china.huawei.com (10.3.19.182) To kwepemm600017.china.huawei.com (7.193.23.234) X-Rspamd-Queue-Id: A2512140003 X-Rspam-User: X-Stat-Signature: jqiswbrzcnwmtr4si3hukrqxy774f1e1 X-Rspamd-Server: rspam01 X-HE-Tag: 1709259214-12644 X-HE-Meta: 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 M2/FmjYf K4V6I7DqOBF1EYW2HORuqVvpsLdnUsMo8u1J/RpC19QCnwIU7sQ/ufT32Ce6GVbLG6FmkEVQ+kneJN8PaEyISObtgqA2um+45Te5vKF6fpJ0cseYM6EtcYMsQ4BttIbx2P7LeeqIn4huMhrXUf+oz/fQ8EtQr3QcmdG1SUYkOSNWVKQaGewj8whW0fjGsKQHxKwZ99d5hqTOPqpiAFQzEiCGPBojyjQFv53JX 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: List-Subscribe: List-Unsubscribe: 在 2024/3/1 1:32, Linus Torvalds 写道: > On Thu, 29 Feb 2024 at 00:13, Tong Tiangen wrote: >> >> See the logic before this patch, always success (((void)(K),0)) is >> returned for three types: ITER_BVEC, ITER_KVEC and ITER_XARRAY. > > No, look closer. > > Yes, the iterate_and_advance() macro does that "((void)(K),0)" to make > the compiler generate better code for those cases (because then the > compiler can see that the return value is a compile-time zero), but > notice how _copy_mc_to_iter() didn't use that macro back then. It used > the unvarnished __iterate_and_advance() exactly so that the MC copy > case would *not* get that "always return zero" behavior. > > That goes back to (in a different form) at least commit 1b4fb5ffd79b > ("iov_iter: teach iterate_{bvec,xarray}() about possible short > copies"). > > But hardly anybody ever tests this machine-check special case code, so > who knows when it broke again. > > I'm just looking at the source code, and with all the macro games it's > *really* hard to follow, so I may well be missing something. > >> Maybe we're all gonna fix it back? as follows： > > No. We could do it for the kvec and xarray case, just to get better > code generation again (not that I looked at it, so who knows), but the > one case that actually uses memcpy_from_iter_mc() needs to react to a > short write. > > One option might be to make a failed memcpy_from_iter_mc() set another > flag in the iter, and then make fault_in_iov_iter_readable() test that > flag and return 'len' if that flag is set. > > Something like that (wild handwaving) should get the right error handling. > > The simpler alternative is maybe something like the attached. > COMPLETELY UNTESTED. Maybe I've confused myself with all the different > indiraction mazes in the iov_iter code. > > Linus Hi Linus: The method in the attachment i have tested before is feasible and can solve this deadloop problem. I also have some confusion about the iov_iter code. Let's take a look at manitainer's comments to see whether there are more comprehensive considerations. Thanks, Tong.