Re: [PATCH v2 net-next 1/6] ch_ktls: Use memcpy_from_page() instead of k[un]map_atomic()

On mercoledì 23 novembre 2022 21:52:14 CET Anirudh Venkataramanan wrote:

kmap_atomic() is being deprecated in favor of kmap_local_page(). Replace
the map-memcpy-unmap usage pattern (done using k[un]map_atomic()) with
memcpy_from_page(), which internally uses kmap_local_page() and
kunmap_local(). This renders the variables 'data' and 'vaddr' unnecessary,
and so remove these too.

Note that kmap_atomic() disables preemption and page-fault processing, but
kmap_local_page() doesn't. When converting uses of kmap_atomic(), one has
to check if the code being executed between the map/unmap implicitly
depends on page-faults and/or preemption being disabled. If yes, then code
to disable page-faults and/or preemption should also be added for
functional correctness. That however doesn't appear to be the case here,
so just memcpy_from_page() is used.

Just two marginal notes:

It looks like you are explaining your mental process and teaching how  
developers should approach these kinds of conversions. Although I'm OK with 
this exposition, a plain assurance that the code being executed between the 
map / unmap did not depend on page-faults and/or preemption being disabled 
would have sufficed. :-)

Ira were suggesting to not use "it appears" and replace with stronger and 
clearer assertions like "it is" or "I can confirm that" (or whatever else like  
these).

As said, no problems at all with regard to the overall goodness of this and 
the other patches.

Also note that the page being mapped is not allocated by the driver, and so
the driver doesn't know if the page is in normal memory. This is the reason
kmap_local_page() is used (via memcpy_from_page()) as opposed to
page_address().

I don't have hardware, so this change has only been compile tested.

Cc: Ayush Sawal <ayush.sawal@chelsio.com>
Cc: Ira Weiny <redacted>
Cc: Fabio M. De Francesco <redacted>
Suggested-by: Fabio M. De Francesco <redacted>
Signed-off-by: Anirudh Venkataramanan <redacted>
---
v1 -> v2:
 Use memcpy_from_page() as suggested by Fabio
 Add a "Suggested-by" tag
 Rework commit message
 Some emails cc'd in v1 are defunct. Drop them. The MAINTAINERS file for
 Chelsio drivers likely needs an update
---
 .../chelsio/inline_crypto/ch_ktls/chcr_ktls.c | 26 +++++++++----------
 1 file changed, 12 insertions(+), 14 deletions(-)

It looks good to me.

Reviewed-by: Fabio M. De Francesco <redacted>

Thanks,

Fabio

prior_data_len);

-				kunmap_atomic(vaddr);
+				memcpy_from_page(prior_data, 

skb_frag_page(f),

+						 skb_frag_off(f) + 

remaining,

skb_frag_page(f),

+						 skb_frag_off(f) + 

remaining,

frag_delta));

frag_delta,

frag_delta);

 			}
 			/* reset tcp_seq as per the prior_data_required 

len */

 			tcp_seq -= prior_data_len;
--
2.37.2