On Thu, Mar 12, 2020 at 04:16:33PM +0000, Steven Price wrote: > > Actually, while you are looking at this, do you think we should be > > adding at least READ_ONCE in the pagewalk.c walk_* functions? The > > multiple references of pmd, pud, etc without locking seems sketchy to > > me. > > I agree it seems worrying. I'm not entirely sure whether the holding of > mmap_sem is sufficient, I looked at this question, and at least for PMD, mmap_sem is not sufficient. I didn't easilly figure it out for the other ones I'm guessing if PMD is not safe then none of them are. > this isn't something that I changed so I've just > been hoping that it's sufficient since it seems to have been working > (whether that's by chance because the compiler didn't generate multiple > reads I've no idea). For walking the kernel's page tables the lack of > READ_ONCE is also not great, but at least for PTDUMP we don't care too much > about accuracy and it should be crash proof because there's no RCU grace > period. And again the code I was replacing didn't have any special > protection. > > I can't see any harm in updating the code to include READ_ONCE and I'm happy > to review a patch. The reason I ask is because hmm's walkers often have this pattern where they get the pointer and then de-ref it (again) then immediately have to recheck the 'again' conditions of the walker itself because the re-read may have given a different value. Having the walker deref the pointer and pass the value it into the ops for use rather than repeatedly de-refing an unlocked value seems like a much safer design to me. If this also implicitly relies on a RCU grace period then it is also missing RCU locking... I also didn't quite understand why walk_pte_range() skipped locking the pte in the no_vma case - I don't get why vma would be related to locking here. I also saw that hmm open coded the pte walk, presumably for performance, so I was thinking of adding some kind of pte_range() callback to avoid the expensive indirect function call per pte, but hmm also can't have the pmd locked... Jason
