linux-kernel - Re: [PATCH 1/3] mm/gup: track FOLL

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Message-ID: <20200127110624.GD19414@quack2.suse.cz>
Date:   Mon, 27 Jan 2020 12:06:24 +0100
From:   Jan Kara <jack@...e.cz>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 1/3] mm/gup: track FOLL_PIN pages

On Fri 24-01-20 18:11:13, John Hubbard wrote:
> +static __maybe_unused struct page *try_grab_compound_head(struct page *page,
> +							  int refs,
> +							  unsigned int flags)
> +{
> +	if (flags & FOLL_GET)
> +		return try_get_compound_head(page, refs);
> +	else if (flags & FOLL_PIN) {
> +		int orig_refs = refs;
> +
> +		/*
> +		 * When pinning a compound page of order > 1 (which is what
> +		 * hpage_pincount_available() checks for), use an exact count to
> +		 * track it, via hpage_pincount_inc/_dec().
> +		 *
> +		 * However, be sure to *also* increment the normal page refcount
> +		 * field at least once, so that the page really is pinned.
> +		 */
> +		if (!hpage_pincount_available(page))
> +			refs *= GUP_PIN_COUNTING_BIAS;
> +
> +		page = try_get_compound_head(page, refs);
> +		if (!page)
> +			return NULL;
> +
> +		if (hpage_pincount_available(page))
> +			hpage_pincount_inc(page);

Umm, adding just 1 to pincount looks dangerous to me as
try_grab_compound_head() would not compose - you could not release
references acquired by try_grab_compound_head() with refs==2 by two calls
to put_compound_head() with refs==1. So I'd rather have here:
hpage_pincount_add(page, refs) and similarly in put_compound_head().
Otherwise the patch looks good to me from a quick look.

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR