> On Mar 23, 2023, at 8:30 PM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: [...] > > The Uptodate flag check needs to be done by the caller; the > find_get_page() family return !uptodate pages. > > But find_get_page() does not advertise itself as NMI-safe. And I > think it's wrong to try to make it NMI-safe. Most of the kernel is > not NMI-safe. I think it's incumbent on the BPF people to get the > information they need ahead of taking the NMI. NMI handlers are not > supposed to be doing a huge amount of work! I don't really understand > why it needs to do work in NMI context; surely it can note the location of > the fault and queue work to be done later (eg on irq-enable, task-switch > or return-to-user) The use case here is a profiler (similar to perf-record). Parsing the build id in side the NMI makes the profiler a lot simpler. Otherwise, we will need some post processing for each sample. OTOH, it is totally fine if build_id_parse() fails some time, say < 5%. The profiler output is still useful in such cases. I guess the next step is to replace find_get_page() with a NMI-safe version? Thanks, Song
