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

From: David Hildenbrand
Date: Wed May 31 2023 - 04:48:14 EST

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

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.


David / dhildenb