David Hildenbrand <david@xxxxxxxxxx> wrote: > > Make pin_user_pages*() leave a ZERO_PAGE unpinned if it extracts a pointer > > to it from the page tables and make unpin_user_page*() correspondingly > > ignore a ZERO_PAGE when unpinning. We don't want to risk overrunning a > > zero page's refcount as we're only allowed ~2 million pins on it - > > something that userspace can conceivably trigger. > > 2 millions pins (FOLL_PIN, which increments the refcount by 1024) or 2 million > references ? Definitely pins. It's tricky because we've been using "pinned" to mean held by a refcount or held by a flag too. 2 million pins on the zero page is in the realms of possibility. It only takes 32768 64-page DIO writes. > > @@ -3079,6 +3096,9 @@ EXPORT_SYMBOL_GPL(get_user_pages_fast); > > * > > * FOLL_PIN means that the pages must be released via unpin_user_page(). Please > > * see Documentation/core-api/pin_user_pages.rst for further details. > > + * > > + * Note that if a zero_page is amongst the returned pages, it will not have > > + * pins in it and unpin_user_page() will not remove pins from it. > > */ > > "it will not have pins in it" sounds fairly weird to a non-native speaker. Oh, I know. The problem is that "pin" is now really ambiguous. Can we change "FOLL_PIN" to "FOLL_NAIL"? Or maybe "FOLL_SCREW" - your pages are screwed if you use DIO and fork at the same time. > "Note that the refcount of any zero_pages returned among the pinned pages will > not be incremented, and unpin_user_page() will similarly not decrement it." That's not really right (although it happens to be true), because we're talking primarily about the pin counter, not the refcount - and they may be separate. David
