> Can you point me to where a refcounted reference to the page comes > from when page_detective_metadata() calls dump_page_lvl()? I am sorry, I remembered incorrectly, we are getting reference right after dump_page_lvl() in page_detective_memcg() -> folio_try_get(); I will move the folio_try_get() to before dump_page_lvl(). > > > So I think dump_page() in its current form is not something we should > > > expose to a userspace-reachable API. > > > > We use dump_page() all over WARN_ONs in MM code where pages might not > > be locked, but this is a good point, that while even the existing > > usage might be racy, providing a user-reachable API potentially makes > > it worse. I will see if I could add some locking before dump_page(), > > or make a dump_page variant that does not do dump_mapping(). > > To be clear, I am not that strongly opposed to racily reading data > such that the data may not be internally consistent or such; but this > is a case of racy use-after-free reads that might end up dumping > entirely unrelated memory contents into dmesg. I think we should > properly protect against that in an API that userspace can invoke. > Otherwise, if we race, we might end up writing random memory contents > into dmesg; and if we are particularly unlucky, those random memory > contents could be PII or authentication tokens or such. > > I'm not entirely sure what the right approach is here; I guess it > makes sense that when the kernel internally detects corruption, > dump_page doesn't take references on pages it accesses to avoid > corrupting things further. If you are looking at a page based on a > userspace request, I guess you could access the page with the > necessary locking to access its properties under the normal locking > rules? I will take reference, as we already do that for memcg purpose, but have not included dump_page(). Thank you, Pasha
