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[209.85.218.47]) by smtp.gmail.com with ESMTPSA id mm27-20020a170906cc5b00b009929ab17be0sm2199218ejb.162.2023.10.18.12.19.00 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 18 Oct 2023 12:19:01 -0700 (PDT) Received: by mail-ej1-f47.google.com with SMTP id a640c23a62f3a-9be7e3fa1daso781732966b.3 for ; Wed, 18 Oct 2023 12:19:00 -0700 (PDT) X-Received: by 2002:a17:906:99c4:b0:9bd:a662:c066 with SMTP id s4-20020a17090699c400b009bda662c066mr149569ejn.76.1697656719721; Wed, 18 Oct 2023 12:18:39 -0700 (PDT) MIME-Version: 1.0 References: <20231018-mgtime-v1-0-4a7a97b1f482@kernel.org> <20231018-mgtime-v1-2-4a7a97b1f482@kernel.org> In-Reply-To: <20231018-mgtime-v1-2-4a7a97b1f482@kernel.org> From: Linus Torvalds Date: Wed, 18 Oct 2023 12:18:22 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH RFC 2/9] timekeeping: new interfaces for multigrain timestamp handing To: Jeff Layton Cc: Alexander Viro , Christian Brauner , John Stultz , Thomas Gleixner , Stephen Boyd , Chandan Babu R , "Darrick J. Wong" , Dave Chinner , "Theodore Ts'o" , Andreas Dilger , Chris Mason , Josef Bacik , David Sterba , Hugh Dickins , Andrew Morton , Amir Goldstein , Jan Kara , David Howells , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-mm@kvack.org, linux-nfs@vger.kernel.org Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 18721A0022 X-Stat-Signature: b8jazkjhubg7z9r19ke13swr3wu4k48s X-Rspam-User: X-HE-Tag: 1697656742-537593 X-HE-Meta: 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 HJp3IU4Z 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 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, 18 Oct 2023 at 10:41, Jeff Layton wrote: > > One way to prevent this is to ensure that when we stamp a file with a > fine-grained timestamp, that we use that value to establish a floor for > any later timestamp update. I'm very leery of this. I don't like how it's using a global time - and a global fine-grained offset - when different filesystems will very naturally have different granularities. I also don't like how it's no using that global lock. Yes, yes, since the use of this all is then gated by the 'is_mgtime()' thing, any filesystem with big granularities will presumably never set FS_MGTIME in the first time, and that hides the worst pointless cases. But it still feels iffy to me. Also, the whole current_ctime() logic seems wrong. Later (in 4/9), you do this: static struct timespec64 current_ctime(struct inode *inode) { if (is_mgtime(inode)) return current_mgtime(inode); and current_mgtime() does if (nsec & I_CTIME_QUERIED) { ktime_get_real_ts64(&now); return timestamp_truncate(now, inode); } so once the code has set I_CTIME_QUERIED, it will now use the expensive fine-grained time - even when it makes no sense. As far as I can tell, there is *never* a reason to get the fine-grained time if the old inode ctime is already sufficiently far away. IOW, my gut feel is that all this logic should always not only be guarded by FS_MGTIME (like you do seem to do), *and* by "has anybody even queried this time" - it should *also* always be guarded by "if I get the coarse-grained time, is that sufficient?" So I think the current_ctime() logic should be something along the lines of static struct timespec64 current_ctime(struct inode *inode) { struct timespec64 ts64 = current_time(inode); unsigned long old_ctime_sec = inode->i_ctime_sec; unsigned int old_ctime_nsec = inode->i_ctime_nsec; if (ts64.tv_sec != old_ctime_sec) return ts64; /* * No need to check is_mgtime(inode) - the I_CTIME_QUERIED * flag is only set for mgtime filesystems */ if (!(old_ctime_nsec & I_CTIME_QUERIED)) return ts64; old_ctime_nsec &= ~I_CTIME_QUERIED; if (ts64.tv_nsec > old_ctime_nsec + inode->i_sb->s_time_gran) return ts64; /* Ok, only *now* do we do a finegrained value */ ktime_get_real_ts64(&ts64); return timestamp_truncate(ts64); } or whatever. Make it *very* clear that the finegrained timestamp is the absolute last option, after we have checked that the regular one isn't possible. I dunno. Linus