Question

Just wondering if the key to shared memory is the file name or the inode.

I have a file called .last, which is just a hard link to a file named YYYYMMDDHHMMSS.

A directory looks like this:

20110101143000  
.last

.last is just a hard link to 20110101143000.

Some time later, a new file is created

20110101143000  
20110622083000  
.last

We then delete .last, and recreate it to refer to the new file.

Our software, which is continuously running during these updates, mmaps the .last file with MAP_SHARED. When done with a file, the software might cache it for several minutes rather than unmap it. On a physical server, there are 12-24 instances of the software running at the same time. Different instances often mmap the same file at about the same time. My question is:

Does linux use the file name to key to the shared memory, or does it use the inode?

Given this scenario:

1. proc A mmaps .last, and does not unmap
2. a new file is written, .last is deleted, a new .last is created to link the new
   file
3. proc B mmaps the new .last, and does not unmap

If linux used the inode, then proc A and B would be seeing different blocks of memory mapped to different files, which is what we want. If linux uses the filename, then both A and B see the same block of memory mapped to the new file. B is fine, but A crashes when the memory in the shard block changes.

Anyone know how it actually works? I’m going to test, but if it turns out to be name based, i am screwed unless someone knows a trick.

Thanks!

Answer 1

It’s the inode, at least effectively. That is to say that once you have mapped some pages from a file they will continue to refer to that file and won’t change just because the mapping of names to files changes in the filesystem.