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[70.52.229.124]) by smtp.gmail.com with ESMTPSA id 11-20020a37030b000000b00745a55db5a3sm12310243qkd.24.2023.03.28.15.34.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 28 Mar 2023 15:34:02 -0700 (PDT) Date: Tue, 28 Mar 2023 18:34:01 -0400 From: Peter Xu To: Axel Rasmussen Cc: Alexander Viro , Andrew Morton , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] userfaultfd: don't fail on unrecognized features Message-ID: References: <20220722201513.1624158-1-axelrasmussen@google.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Stat-Signature: 9twu5hng3hbqd73z1xxsweqwzae98u4q X-Rspam-User: X-Rspamd-Queue-Id: 5D782C0015 X-Rspamd-Server: rspam06 X-HE-Tag: 1680042846-941217 X-HE-Meta: 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 NKyyHPbn rB1VwklVvP9BAxJS4+fpxy+L/Byz35XZtZD/mWsOMmL8l5vtcBaHw5lUmaZN+pwR60XyraG4+rDz9QrF2Jn511i5L6dL2u48sPc81Bwa1b+aSVBa6J2a9AdP2zFCEYi/n5WH2M0DVJ8JQuBYGEK82dPBjw9ZC8sMpuH+uD5XIzllhkCmq1E7qXL1lYJqfjMwaoOgBGdSBVnE2LUGOEtggMdOuH4OWqwhODbZR6i2iBokpB8hoIJfbqeSucww29Wns/jSdK2SaZgR2K4pHFUbKCZVfDbUK6Kehosc+t3gKhpxlfnl9x0T2WhWlhtWiOYCu48+FQwe3gqwF/+7GZhKYoWOdgZpvQ8cDnVbxQzqM9Uab7PICcpxH50mRNA== 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 Tue, Mar 28, 2023 at 02:52:35PM -0700, Axel Rasmussen wrote: > I don't see being very strict here as useful. Another example might be > madvise() - for example trying to MADV_PAGEOUT on a kernel that > doesn't support it. There is no way the kernel can proceed here, since > it simply doesn't know how to do what you're asking for. In this case > an error makes sense. IMHO, PAGEOUT is not a great example. I wished we can have a way to probe what madvise() the system supports, and I know many people wanted that too. I even had a feeling that we'll have it some day. So now I'm going back to look at this patch assuming I'm reviewing it, I'm still not convinced the old API needs changing. Userfaultfd allows probing with features=0 with/without this patch, so I see this patch as something that doesn't bring a direct functional benefit, but some kind of api change due to subjective preferences which I cannot say right or wrong. Now the patch is already merged. If we need to change either this patch or the man page to make them match again, again I'd prefer we simply revert it to keep everything like before and copy stable. Thanks, -- Peter Xu