linux-kernel - Re: [PATCHv7 2/3] mm/gup: fix omission of check on FOLL

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Message-ID: <20200317120930.GA435@infradead.org>
Date:   Tue, 17 Mar 2020 05:09:30 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Pingfan Liu <kernelfans@...il.com>
Cc:     linux-mm@...ck.org, Ira Weiny <ira.weiny@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        John Hubbard <jhubbard@...dia.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Christoph Hellwig <hch@...radead.org>,
        Shuah Khan <shuah@...nel.org>, Jason Gunthorpe <jgg@...pe.ca>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv7 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in
 gup fast path

On Tue, Mar 17, 2020 at 07:47:32PM +0800, Pingfan Liu wrote:
> FOLL_LONGTERM is a special case of FOLL_PIN. It suggests a pin which is
> going to be given to hardware and can't move. It would truncate CMA
> permanently and should be excluded.
> 
> In gup slow path, slow path, where
> __gup_longterm_locked->check_and_migrate_cma_pages() handles FOLL_LONGTERM,
> but in fast path, there lacks such a check, which means a possible leak of
> CMA page to longterm pinned.
> 
> Place a check in try_grab_compound_head() in the fast path to fix the leak,
> and if FOLL_LONGTERM happens on CMA, it will fall back to slow path to
> migrate the page.
> 
> Some note about the check:
> Huge page's subpages have the same migrate type due to either
> allocation from a free_list[] or alloc_contig_range() with param
> MIGRATE_MOVABLE. So it is enough to check on a single subpage
> by is_migrate_cma_page(subpage)

Looks good,

Reviewed-by: Christoph Hellwig <hch@....de>