On 07/20/2017 10:45 AM, Miroslav Benes wrote:
>
>>>>> + *
>>>>> + * Note: allocates @new_size space for shadow variable data and copies
>>>>> + * @new_size bytes from @new_data into the shadow varaible's own @new_data
>>>>> + * space. If @new_data is NULL, @new_size is still allocated, but no
>>>>> + * copy is performed.
>>>>
>>>> I must say I'm not entirely happy with this. I don't know if this is what
>>>> Petr had in mind (I'm sure he'll get to the patch set soon). Calling
>>>> memcpy instead of a simple assignment in v1 seems worse.
>>>
>>> This change was a bit of a experiment on my part in reaction to
>>> adding klp_shadow_get_or_attach().
>>>
>>> I like the simplicity of v1's pointer assignment -- in fact, moving all
>>> allocation responsiblity (klp_shadow meta-data and data[] area) out to
>>> the caller is doable, though implementing klp_shadow_get_or_attach() and
>>> and klp_shadow_detach_all() complicates matters, for example, adding an
>>> alloc/release callback. I originally attempted this for v2, but turned
>>> back when the API and implementation grew complicated. If the memcpy
>>> and gfp_flag restrictions are too ugly, I can try revisting that
>>> approach. Ideas welcome :)
>>
>> Well, I didn't like callbacks either :). And no, I do not have a better
>> idea. I still need to think about it.
>
> Done and I agree that memcpy approach is not so bad after all :). So I'm
> fine with it.
I looked at it again this morning and a "pass-your-own" allocation API
always comes back to adding callbacks and other complications :( In the
end, most callers will be shadowing pointers and not entire structures,
so I think the copy isn't too bad.
On a related note, if we keep the allocations and memcpy, how about I
shift around the attach/get calls like so:
__klp_shadow_attach
set shadow variable member values
memcpy
add to hash
klp_shadow_attach
alloc new shadow var
lock
call __klp_shadow_attach with new alloc
unlock
klp_shadow_get_or_attach
be optimistic, call klp_shadow_get (if found, return it)
be pessimistic, alloc new shadow var
lock
call klp_shadow_get again
if unlikely found
kfree unneeded alloc
else
call __klp_shadow_attach with new alloc
unlock
return whichever shadow var we used
This way both calls can accept gfp_flags that may sleep, with the only
downside that klp_shadow_get_or_attach may allocate an unnecessary
shadow variable in the unlikely case that it's found on the second
klp_shadow_get attempt (under the lock). No more clunky "bool lock"
flag either. :)
-- Joe
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