Why crypto api only in user context?

Hi,

I'm changing a little kernel code as part of a research on memory
cards. I'm trying to use the api to get page hashes for frames. I read
in kernel doc that it only can be used in user context or softirq but
I need to get the hash in the page fault handler. Im trying to find
out the reason of that api restriction in order to decide what to do.
My options are trying to reuse only part of the api, deferring my work
out of the exc. handler or as a last resource designing my own hash
api without this restrictions.

The questions:

Why the restriction?
Using directly md5 or sha1 implementations solves the problem?
More usefull Doc?


Thanks in advance.

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Linux-crypto:  cryptography in and on the Linux system
Archive:       http://mail.nl.linux.org/linux-crypto/