Re: [PATCH v4 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages()

[David Hildenbrand]

On 31.05.23 10:35, David Howells wrote:
> 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.

Yes, it would be clearer if we would be using "pinned" now only for 
FOLL_PIN and everything else is simply "taking a temporary reference on 
the page".

> 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.

I'm hoping that "pinning" will be "FOLL_PIN" (intention to access page 
content) and everything else is simply "taking a temporary page reference".

> > "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.

In any case (FOLL_PIN/FOLL_GET) you increment/decrement the refcount. If 
we have a separate pincount, we increment/decrement the refcount by 1 
when (un)pinning.

Sure, if we'd have a separate pincount we'd also not be modifying it.

--
Thanks,

David / dhildenb