linux-kernel - Re: [PATCH v3 1/6] mm, gup: Remove broken VM_BUG_ON

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Message-ID: <20170523130947.cv3bbjxa2l4ifj55@node.shutemov.name>
Date:   Tue, 23 May 2017 16:09:47 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Punit Agrawal <punit.agrawal@....com>
Cc:     akpm@...ux-foundation.org, Will Deacon <will.deacon@....com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, catalin.marinas@....com,
        n-horiguchi@...jp.nec.com, kirill.shutemov@...ux.intel.com,
        mike.kravetz@...cle.com, steve.capper@....com,
        mark.rutland@....com, hillf.zj@...baba-inc.com,
        linux-arch@...r.kernel.org, aneesh.kumar@...ux.vnet.ibm.com
Subject: Re: [PATCH v3 1/6] mm, gup: Remove broken VM_BUG_ON_PAGE compound
 check for hugepages

On Mon, May 22, 2017 at 02:35:59PM +0100, Punit Agrawal wrote:
> From: Will Deacon <will.deacon@....com>
> 
> When operating on hugepages with DEBUG_VM enabled, the GUP code checks the
> compound head for each tail page prior to calling page_cache_add_speculative.
> This is broken, because on the fast-GUP path (where we don't hold any page
> table locks) we can be racing with a concurrent invocation of
> split_huge_page_to_list.
> 
> split_huge_page_to_list deals with this race by using page_ref_freeze to
> freeze the page and force concurrent GUPs to fail whilst the component
> pages are modified. This modification includes clearing the compound_head
> field for the tail pages, so checking this prior to a successful call
> to page_cache_add_speculative can lead to false positives: In fact,
> page_cache_add_speculative *already* has this check once the page refcount
> has been successfully updated, so we can simply remove the broken calls
> to VM_BUG_ON_PAGE.
> 
> Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> Cc: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
> Signed-off-by: Will Deacon <will.deacon@....com>
> Acked-by: Steve Capper <steve.capper@....com>
> Signed-off-by: Punit Agrawal <punit.agrawal@....com>

Looks reasonable to me:

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

-- 
 Kirill A. Shutemov