linux-kernel - Re: [PATCH v4 1/3] mm: Don't pin ZERO_PAGE in pin_user

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Message-ID: <492558dc-1377-fc4b-126f-c358bb000ff7@redhat.com>
Date:   Wed, 31 May 2023 10:46:30 +0200
From:   David Hildenbrand <david@...hat.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Jens Axboe <axboe@...nel.dk>,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Hillf Danton <hdanton@...a.com>,
        Christian Brauner <brauner@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v4 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages()

On 31.05.23 10:35, David Howells wrote:
> David Hildenbrand <david@...hat.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