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    SubjectRe: [PATCH 01/04] Adding cipher mode context information to crypto_tfm
    Fruhwirth Clemens <> wrote:
    > On Wed, 2005-02-09 at 17:19 -0800, Andrew Morton wrote:
    > > Fruhwirth Clemens <> wrote:
    > > >
    > > > It must be
    > > > possible to process more than 2 mappings in softirq context.
    > >
    > > Adding a few more fixmap slots wouldn't hurt anyone. But if you want an
    > > arbitrarily large number of them then no, we cannot do that.
    > What magnitude is "few more"? 2, 10, 100?

    Not 100. 10 would seem excessive.

    > > Taking more than one sleeping kmap at a time within the same process is
    > > deadlocky, btw. You can end up with N such tasks all holding one kmap and
    > > waiting for someone else to release one.
    > >
    > > Possibly one could arrange for the pages to not be in highmem at all.
    > Is there an easy way to bring pages to lowmem? The cryptoapi is called
    > from the backlog of the networking stack, which is assigned in irq
    > context first and processed softirq context.

    Are networking frames ever allocated from highmem? Don't think so.

    > There is little opportunity
    > to bringt stuff to low mem. And we can't bringt stuff to lowmem on our
    > own as well, because (as I guess) this involves a page allocation, which
    > would have to be GFP_ATOMIC, which can fail. So we would need fallback
    > for the fallback mechanism, and that's still the tiny set of scratch
    > buffers.. hairy business..


    > Ok, what about the following plan:
    > If context == softirq, use kmap_atomic until they all used, fall-back to
    > scratch buffers, and printk in some DEBUG mode:"Warning slow, redesign
    > your client or raise the number of fixmaps".

    Sounds painful.

    > If context == user, use kmap_atomic until they all used, and fall-back
    > to kmap.

    Taking multiple kmaps can deadlock due to kmap exhaustion though.
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