Messages in this thread | | | Date | Wed, 31 May 2023 10:46:30 +0200 | From | David Hildenbrand <> | Subject | Re: [PATCH v4 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages() |
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On 31.05.23 10:35, David Howells wrote: > David Hildenbrand <david@redhat.com> 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
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