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

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Date:   Thu, 29 Sep 2022 14:25:16 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
CC:     Muchun Song <songmuchun@...edance.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 2022/9/15 6:18, 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.

As previous thread [1] points out, if vma->vm_private_data is NULL, there won't be any synchronization
which provides the same type of synchronization around i_size as provided by the fault mutex as in [2].
([2] will take the fault mutex for EVERY index in the truncated range)

[1] https://lore.kernel.org/lkml/YyOKIhygl66cG8Yr@monkey/T/#m6b69af9e8cdba01246c2b210bd044bf895b815ee
[2] https://lore.kernel.org/lkml/20220824175757.20590-5-mike.kravetz@oracle.com/

Except from that, this patch looks good to me. Thanks Mike.

Thanks,
Miaohe Lin