Volatile ranges with fallocate() [LWN.net]

Volatile ranges with fallocate()

Posted Jun 8, 2012 0:31 UTC (Fri) by robert_s (subscriber, #42402)
Parent article: Volatile ranges with fallocate()

"Any filesystem could conceivably implement this functionality, but, in practice, it only makes sense for a RAM-based filesystem like tmpfs, so it is only implemented there."

So, for the stated example, the web browser would have to:

1. Find a mounted ram-based filesystem it had access to.
2. Create a file for the cache.
3. Presumably mmap() the file so it can be accessed conveniently.

Also, how would the application know when an area had been reclaimed? Canaries? What if an application tried to access an object that had only partially been reclaimed - say a single 4k page had innocuously been taken from a huge blob?

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Volatile ranges with fallocate()

Posted Jun 8, 2012 0:35 UTC (Fri) by dlang (✭ supporter ✭, #313) [Link]

to answer your second issue (how would it know that it's been reclaimed), per the article, the application does

fallocate(fd, FALLOCATE_FL_UNMARK_VOLATILE, offset, len);

and if the return code is 0, it can use the data, if the return code is non-zero, some part of the data has been reclaimed and the application needs to regenerate it.

I would hope that when the system grabs part of a block that's been marked volatile, it grabs the entire block, rather then just punching a small hole in it (or at least remembers what it damaged earlier when it needs more memory, on the theory that you may mark a large block volitile and then try to re-use small chunks of it)