linux-kernel - Re: [RFC PATCH v2 1/3] mm: Don't pin ZERO_PAGE in pin_user

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Date:   Fri, 26 May 2023 10:15:26 +0100
From:   David Howells <dhowells@...hat.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     dhowells@...hat.com, Christoph Hellwig <hch@...radead.org>,
        David Hildenbrand <david@...hat.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: [RFC PATCH v2 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages()

Lorenzo Stoakes <lstoakes@...il.com> wrote:

> > iov_iter_extract_pages(), on the other hand, is only used in two places
> > with these patches and the pins are always released with
> > unpin_user_page*() so it's a lot easier to audit.
> 
> Thanks for the clarification. I guess these are the cases where you're
> likely to see zero page usage, but since this is changing all PUP*() callers
> don't you need to audit all of those too?

I don't think it should be necessary.  This only affects pages obtained from
gup with FOLL_PIN - and, so far as I know, those always have to be released
with unpin_user_page*() which is part of the gup API and thus it should be
transparent to the users.

Pages obtained FOLL_GET, on the other hand, aren't freed through the gup API -
and there are a bunch of ways of releasing them - and getting additional refs
too.

David