From: David Hildenbrand <david@redhat.com>
To: David Howells <dhowells@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>,
Lorenzo Stoakes <lstoakes@gmail.com>,
Jens Axboe <axboe@kernel.dk>, Al Viro <viro@zeniv.linux.org.uk>,
Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
Jeff Layton <jlayton@kernel.org>,
Jason Gunthorpe <jgg@nvidia.com>,
Logan Gunthorpe <logang@deltatee.com>,
Hillf Danton <hdanton@sina.com>,
Christian Brauner <brauner@kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v4 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages()
Date: Wed, 31 May 2023 10:46:30 +0200 [thread overview]
Message-ID: <492558dc-1377-fc4b-126f-c358bb000ff7@redhat.com> (raw)
In-Reply-To: <510965.1685522152@warthog.procyon.org.uk>
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
next prev parent reply other threads:[~2023-05-31 8:46 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-26 21:41 [PATCH v4 0/3] block: Make old dio use iov_iter_extract_pages() and page pinning David Howells
2023-05-26 21:41 ` [PATCH v4 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages() David Howells
2023-05-27 19:46 ` Lorenzo Stoakes
2023-05-31 3:57 ` Christoph Hellwig
2023-05-31 7:34 ` David Hildenbrand
2023-05-31 8:35 ` David Howells
2023-05-31 8:46 ` David Hildenbrand [this message]
2023-05-31 13:55 ` David Howells
2023-05-31 14:10 ` Lorenzo Stoakes
2023-05-26 21:41 ` [PATCH v4 2/3] mm: Provide a function to get an additional pin on a page David Howells
2023-05-31 3:57 ` Christoph Hellwig
2023-05-31 7:42 ` David Hildenbrand
2023-05-31 8:20 ` David Howells
2023-05-26 21:41 ` [PATCH v4 3/3] block: Use iov_iter_extract_pages() and page pinning in direct-io.c David Howells
2023-05-31 3:58 ` Christoph Hellwig
2023-05-31 15:48 ` [PATCH v4 0/3] block: Make old dio use iov_iter_extract_pages() and page pinning Jens Axboe
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