How to identify a 'fresh' page from read_cache_page?

[Pranay Srivastava]

On Jan 28, 2014 7:57 AM, "Zameer Manji" <zmanji at 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

If readpage is called then it's because

a) page not in page cache

b) page isn't uptodate.

`readpage` with a page that we have not written to before (since the user
is increasing the file size via `ftruncate`).

I'm  not so sure but is it calling truncate on inode mapping?
Since that would invalidate your page cache pages.

>
> 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?

PageUptodate(page) is your best bet.

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 pagewith garbage data).

Fresh page would not have uptodate flag set which would.be set in readpage
implementation.

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
>
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     ---P.K.S
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