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 09:43:35 +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:

> I guess we're not quite as concerned about FOLL_GET because FOLL_GET should
> be ephemeral and FOLL_PIN (horrifically) adds GUP_PIN_COUNTING_BIAS each
> time?

It's not that - it's that iov_iter_get_pages*() is a lot more commonly used at
the moment, and we'd have to find *all* the places that things using that hand
refs around.

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.

I could modify put_page(), folio_put(), etc. to ignore the zero pages, but
that might have a larger performance impact.

> > +		if (is_zero_page(page))
> > +			return page_folio(page);
> > +
> 
> This will capture huge page cases too which have folio->_pincount and thus
> don't suffer the GUP_PIN_COUNTING_BIAS issue, however it is equally logical
> to simply skip these when pinning.

I'm not sure I understand.  The zero page(s) is/are single-page folios?

> This does make me think that we should just skip pinning for FOLL_GET cases
> too - there's literally no sane reason we should be pinning zero pages in
> any case (unless I'm missing something!)

As mentioned above, there's a code auditing issue and a potential performance
issue, depending on how it's done.

> Another nitty thing that I noticed is, in is_longterm_pinnable_page():-
> 
> 	/* The zero page may always be pinned */
> 	if (is_zero_pfn(page_to_pfn(page)))
> 		return true;
> 
> Which, strictly speaking I suppose we are 'pinning' it or rather allowing
> the pin to succeed without actually pinning, but to be super pedantic
> perhaps it's worth updating this comment too.

Yeah.  It is "pinnable" but no pin will actually be added.

David