On 1/24/14, Zameer Manji <zmanji@gmail.com> wrote:

Hey,

My colleague Will and I are working on improving eCryptfs, an encrypted file system that ships with linux. We are trying to add a new cipher mode and we have run into a problem [1]. When the user calls `ftruncate` on a file and increases the file size, eCryptfs attempts fetch new pages for the file by calling `read_mapping_page` and which calls `read_cache_page`. This calls eCryptfs' `readpage` implementation. We believe `read_cache_page` calls `readpage` with a page that we have not written to before (since the user is increasing the file size via `ftruncate`).

What function can we use to identify when we are given a page to our `readpage` implementation that is a page we have never written to before?

Why do you think that will happen? I mean why would you get a page in your readpage implmentation that you already wrote? Unless you unlock the recently written page calling thread would wait for it. Be it a thread or reclaim by VM. As long as your readpage doesn't want to touch a page which is being handled by another readpage call already i think you are safe to assume that your readpage is getting only a NOT_UPTODATE page.

We need to do this so we know if should check the integrity of the data in the page (if we wrote to it before) or just ignore the contents (because it is a fresh page with garbage data). For more information our ecryptfs_readpage implementation is available on github [2].

Please note that I am not subscribed to kernelnewbies so please include me directly in any replies.

[1]: http://marc.info/?l=ecryptfs-users&m=139007357027191&w=2 [2]: https://github.com/zmanji/ecryptfs/blob/next-patch/fs/ecryptfs/mmap.c#L193

-- Zameer Manji

-- Pranay Srivastava