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Mon, 20 Apr 2026 07:59:46 -0700 (PDT) MIME-Version: 1.0 References: <20260418163358.2304490-1-luca.boccassi@gmail.com> <20260418163358.2304490-4-luca.boccassi@gmail.com> <20260420-unbeeindruckt-besprach-910fd241c32e@brauner> In-Reply-To: From: Luca Boccassi Date: Mon, 20 Apr 2026 15:59:35 +0100 X-Gm-Features: AQROBzCDWEaOuMvM8YwyjtBG72XsuHWKgq2-c-ABNz3Xm8hWnI2pTD0W_boZwEc Message-ID: Subject: Re: [PATCH v8 3/6] liveupdate: add LUO_SESSION_MAGIC magic inode type To: Pasha Tatashin Cc: Christian Brauner , kexec@lists.infradead.org, linux-mm@kvack.org, graf@amazon.com, rppt@kernel.org, pratyush@kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam10 X-Stat-Signature: pukahz5kjzkqzh9xttymyasi1xnrrbf3 X-Rspam-User: X-Rspamd-Queue-Id: AD1521A000D X-HE-Tag: 1776697187-80001 X-HE-Meta: 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 pbXIAlzk 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, 20 Apr 2026 at 15:55, Pasha Tatashin wrote: > > On 04-20 14:26, Christian Brauner wrote: > > On Sat, Apr 18, 2026 at 05:28:20PM +0100, luca.boccassi@gmail.com wrote: > > > From: Luca Boccassi > > > { > > > - char name_buf[128]; > > > + char name_buf[LIVEUPDATE_SESSION_NAME_LENGTH + 1]; > > > struct file *file; > > > > > > lockdep_assert_held(&session->mutex); > > > - snprintf(name_buf, sizeof(name_buf), "[luo_session] %s", session->name); > > > - file = anon_inode_getfile(name_buf, &luo_session_fops, session, O_RDWR); > > > - if (IS_ERR(file)) > > > + > > > + ihold(luo_session_inode); > > > > Right, you're now sharing the same inode among all luo sessions. So > > you've gained the ability to recognize luo inodes via fstatfs() but you > > still can't compare two luo session file descriptors for equality using > > stat() which is a major win and if you're doing this work anyway, let's > > Luca, is there a specific use case in userspace where we need to compare > LUO sessions for equality? > > Christian's proposed solution of using unique inodes provides a standard > VFS interface, but it introduces some memory overhead and, more > importantly, a performance overhead due to the extra metadata > allocations required during the performance-critical kexec blackout > window. > > I do not mind adding it, but I want to verify if this equality check is > actually needed for your userspace agents, once we extend this, there is > no way back. I don't have any use for this comparison as of now, and cannot think of any future one either. So I agree with you and I think we should postpone until there's an actual use case for it, to avoid the downsides.