On 2022-12-06 20:12:13 [+0100], Fabio M. De Francesco wrote: > > Furthermore, code between the kmap_atomic() and kunmap_atomic() > > functions may implicitly depended > > I suppose it should be "depend"? Shouldn't it? Ehm, yes, correct. > > on the side effects of kmap_atomic() > > namely disabling pagefaults or preemption or both. > > I agree with you for rephrasing, mainly because it is > written in poor English. > > However, I still have doubts about why you deleted "migration". > AFAIK, __kmap_local_pfn_prot() always takes care of disabling migration for > HIGHMEM enabled kernels. That is correct. Historically kmap_atomic() never had a migrate_disable() statement - only preempt_disable(). With disabled preemption the task migration is implicitly disabled. > How about !HIGHMEM, where kmap_local_page() is an indirect call to > page_address()? Did you mean that, if the code between kmap_atomic() and > kunmap_atomic() depended on migrate_disable() (in PREEMPT_RT) we should always > just stay safe and call preempt_disable() together with conversion to > kmap_local_page()? Even in the !HIGHMEM case it always uses preempt_disable(). With PREEMPT_RT it is different as it never disabled preemption and always did a migrate_disable() instead. If you talk about what needs to be considered while migrating away from kmap_atomic() then I wouldn't add the PREEMPT_RT bits to it since it was never in the picture while the code (using kmap_atomic()) was originally written. > If so, I understand and I again agree with you. If not, I'm missing something; > so please let me understand properly. > > Aside from the above, I'm not sure whether you deleted the last phrase before > your suggestion. What about making it to become "For the above-mentioned > cases, conversions should also explicitly disable page-faults and/or > preemption"? They need to disable preemption or page-faults or both if it is needed (not unconditionally) and where it is needed. This means not unconditionally over the whole kmap-ed section. > Thanks again for noticing my mistakes. > > Fabio Sebastian
