linux-kernel - Re: [PATCH v2 9/9] hugetlb: clean up code checking for fault/truncation races

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Date:   Mon, 19 Sep 2022 16:32:21 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     Muchun Song <songmuchun@...edance.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        David Hildenbrand <david@...hat.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Michal Hocko <mhocko@...e.com>, Peter Xu <peterx@...hat.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Prakash Sangappa <prakash.sangappa@...cle.com>,
        James Houghton <jthoughton@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Ray Fucillo <Ray.Fucillo@...ersystems.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 9/9] hugetlb: clean up code checking for
 fault/truncation races

On 09/14/22 15:18, Mike Kravetz wrote:
> With the new hugetlb vma lock in place, it can also be used to handle
> page fault races with file truncation.  The lock is taken at the
> beginning of the code fault path in read mode.  During truncation, it
> is taken in write mode for each vma which has the file mapped.  The
> file's size (i_size) is modified before taking the vma lock to unmap.
> 
> How are races handled?
> 
> The page fault code checks i_size early in processing after taking the
> vma lock.  If the fault is beyond i_size, the fault is aborted.  If the
> fault is not beyond i_size the fault will continue and a new page will
> be added to the file.  It could be that truncation code modifies i_size
> after the check in fault code.  That is OK, as truncation code will soon
> remove the page.  The truncation code will wait until the fault is
> finished, as it must obtain the vma lock in write mode.
> 
> This patch cleans up/removes late checks in the fault paths that try to
> back out pages racing with truncation.  As noted above, we just let the
> truncation code remove the pages.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> ---
>  fs/hugetlbfs/inode.c | 31 ++++++++++++-------------------
>  mm/hugetlb.c         | 27 ++++++---------------------
>  2 files changed, 18 insertions(+), 40 deletions(-)

This patch introduced a compiler warning addressed here,

https://lore.kernel.org/linux-mm/Yyj7HsJWfHDoU24U@monkey/

-- 
Mike Kravetz