linux-kernel - Re: [PATCH v3 08/12] mm/gup: page->hpage_pinned_refcount: exact pin counts for huge pages

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Message-ID: <20200203134529.onxociznb5mgtjhf@box>
Date:   Mon, 3 Feb 2020 16:45:29 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
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>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        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>
Subject: Re: [PATCH v3 08/12] mm/gup: page->hpage_pinned_refcount: exact pin
 counts for huge pages

On Fri, Jan 31, 2020 at 07:40:25PM -0800, John Hubbard wrote:
> For huge pages (and in fact, any compound page), the
> GUP_PIN_COUNTING_BIAS scheme tends to overflow too easily, each tail
> page increments the head page->_refcount by GUP_PIN_COUNTING_BIAS
> (1024). That limits the number of huge pages that can be pinned.
> 
> This patch removes that limitation, by using an exact form of pin
> counting for compound pages of order > 1. The "order > 1" is required
> because this approach uses the 3rd struct page in the compound page, and
> order 1 compound pages only have two pages, so that won't work there.

Could you update the comment for HPAGE_PMD_ORDER < 2 check in
hugepage_init() to reflect addtional user for the condition.
> 
> A new struct page field, hpage_pinned_refcount, has been added,
> replacing a padding field in the union (so no new space is used).
> 
> This enhancement also has a useful side effect: huge pages and compound
> pages (of order > 1) do not suffer from the "potential false positives"
> problem that is discussed in the page_dma_pinned() comment block. That
> is because these compound pages have extra space for tracking things, so
> they get exact pin counts instead of overloading page->_refcount.
> 
> Documentation/core-api/pin_user_pages.rst is updated accordingly.
> 
> Suggested-by: Jan Kara <jack@...e.cz>
> Signed-off-by: John Hubbard <jhubbard@...dia.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
 Kirill A. Shutemov