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Re: [NFS] [PATCH] Attribute timeout handling and wrapping u32 jiffies
I would like to discuss the idea that the current checks for attribute timeout using time_after are inadequate for 32bit architectures, since time_after works correctly only when the two timestamps being compared are within 2^31 jiffies of each other. The signed overflow caused by comparing values more than 2^31 jiffies apart will flip the result, causing incorrect assumptions of validity. 2^31 jiffies is a fairly large period of time (~25 days) when compared to the lifetime of most kernel data structures, but for long lived NFS mounts that can sit idle for months (think that for some reason autofs cannot be used), it is easy to compare inode attribute timestamps with very disparate or even bogus values (as in when jiffies have wrapped many times, where the comparison doesn't even make sense). Currently the code tests for attribute timeout by simply adding the desired amount of jiffies to the stored timestamp and comparing that with the current timestamp of obtained attribute data with time_after. This is incorrect, as it returns true for the desired timeout period and another full 2^31 range of jiffies. In testing with artificial jumps (several small jumps, not one big crank) of the jiffies I was able to reproduce a problem found in a server with very long lived NFS mounts, where attributes would not be refreshed even after touching files and directories in the server: Initial uptime: 03:42:01 up 6 min, 0 users, load average: 0.01, 0.12, 0.07 NFS volume is mounted and time is advanced: 03:38:09 up 25 days, 2 min, 0 users, load average: 1.22, 1.05, 1.08 # ls -l /local/A/foo/bar /nfs/A/foo/bar -rw-r--r-- 1 root root 0 Dec 17 03:38 /local/A/foo/bar -rw-r--r-- 1 root root 0 Nov 22 00:36 /nfs/A/foo/bar # touch /local/A/foo/bar # ls -l /local/A/foo/bar /nfs/A/foo/bar -rw-r--r-- 1 root root 0 Dec 17 03:47 /local/A/foo/bar -rw-r--r-- 1 root root 0 Nov 22 00:36 /nfs/A/foo/bar We can see the local mtime is updated, but the NFS mount still shows the old value. The patch below makes it work: Initial setup... 07:11:02 up 25 days, 1 min, 0 users, load average: 0.15, 0.03, 0.04 # ls -l /local/A/foo/bar /nfs/A/foo/bar -rw-r--r-- 1 root root 0 Jan 11 07:11 /local/A/foo/bar -rw-r--r-- 1 root root 0 Jan 11 07:11 /nfs/A/foo/bar # touch /local/A/foo/bar # ls -l /local/A/foo/bar /nfs/A/foo/bar -rw-r--r-- 1 root root 0 Jan 11 07:14 /local/A/foo/bar -rw-r--r-- 1 root root 0 Jan 11 07:14 /nfs/A/foo/bar Signed-off-by:Fabio Olive Leite <fleite@redhat.com> Signed-off-by: