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How to open a file without open system call (edit)
Revision as of 08:31, 3 December 2014
, 08:31, 3 December 2014no edit summary
Chances to get the name back exist. To understand when let's dive a little bit more in how Linux manages dentries and inodes.
Chances to get the name back exist. To understand when let's dive a little bit more in how Linux manages dentries and inodes.
=== Inodes and dentries ===
So, every file on disk is represented by an inode object. Inode has an ID (inode number), access rights, owner, link count and some more data. Names are only stored in special files called ''directories'' -- in directories there's a set of name-to-inode mappings. When accessing a file by its name Linux kernel sequentially reads from disk these mapping tables and for every name found in it creates a dentry object in memory. It's important to know, that dentry is created not only for existing files, but also for non-existing to speed up the ENOENT report for second file lookup. IOW dentries form a cache, which contains records for both present and absent objects on disk.
So, every file on disk is represented by an inode object. Inode has an ID (inode number), access rights, owner, link count and some more data. Names are only stored in special files called ''directories'' -- in directories there's a set of name-to-inode mappings. When accessing a file by its name Linux kernel sequentially reads from disk these mapping tables and for every name found in it creates a dentry object in memory. It's important to know, that dentry is created not only for existing files, but also for non-existing to speed up the ENOENT report for second file lookup. IOW dentries form a cache, which contains records for both present and absent objects on disk.
Having said that, at the time the fsnotify creating happens we have a full dentry chain and the inode sitting in memory. Then the events generator is put on the inode and that's it. Neither inode nor fsnotify object references the dentry, so eventually the whole dentry chain can be shrunk from memory.
Having said that, at the time the fsnotify creating happens we have a full dentry chain and the inode sitting in memory. Then the events generator is put on the inode and that's it. Neither inode nor fsnotify object references the dentry, so eventually the whole dentry chain can be shrunk from memory.
So, returning to the "can I get the name back" question. The answer is -- if the dentry cache is still alive -- yes, you can. But CRIU cannot rely on this, since it should also support situations when the dentry cache is not there.
=== Tmpfs ===
One filesystem, however, behaves friendly to this problem. The tempfs one pins the dentries in memory, since it has no other media on which to store the information about files on it. So for tmpfs the name is always at hands.
[[Category: Under the hood]]
[[Category: Under the hood]]