pwmarcz / fs-demo Goto Github PK

How do we plan to tackle the case where two processes:

1. Access the same file
2. At least one of them uses mmap()
3. At least one of them has write permission

Do we consider this case unlikely and thus fail on it? Like, when the second file tries to do an mmap() on the already-opened file (by another process), we return an error?

When a syscall operates on two files, there seems to be a possibility of a deadlock.

A trivial example would be:

1. First process does rename("foo.txt", "bar.txt")
2. Second process does rename("bar.txt", "foo.txt")

Roughly at the same time. Another example would be sendfile().

Or maybe there is no problem actually? The Server can impose some ordering to prevent deadlocks?

From the README and diagrams, it's hard to understand what kind of Server state is kept. Also, it's not obvious what is the relationship between Dentries and Handles.

From what I understand:

• Server keeps all known dentries; they are never removed
  • Each dentry has a canonical path + some file metadata
  • Each dentry has a list of associated handles (? I imagine this is needed to propagate things like "file was removed")
• Server keeps all known handles
  • Handles are removed when closed by all clients
  • Each handle references a corresponding dentry (which may be in "negative" state if file was removed)

Also:

• Several clients may use the same handle (depicted on the second diagram)
  • All their accesses to this handle will be synchronized by the server (including the position pointer)
• Several clients may use two handles backed by the same dentry (not depicted on diagrams, but I guess similar to the first diagram)
  • All their accesses to these handles with the same underlying dentry will be synchronized by the server (but not the position pointer)
• Clients that use handles backed by different dentries are never synchronized by the server (except for corner cases of rename and sendfile and maybe some more).

Is this understanding correct?

I found Sprite which may contain some ideas for you:

• https://en.wikipedia.org/wiki/Sprite_(operating_system)#:~:text=Sprite%20is%20an%20experimental%20Unix,of%20the%20log%2Dstructured%20filesystem.
• https://users.soe.ucsc.edu/~sbrandt/290S/sprite.pdf

One interesting aspect of Sprite is the use of version numbers for cache consistency on clients.

Hope this helps.