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

From: Miaohe Lin <linmiaohe@huawei.com>
To: Mike Kravetz <mike.kravetz@oracle.com>, <linux-mm@kvack.org>,
	<linux-kernel@vger.kernel.org>
Cc: Muchun Song <songmuchun@bytedance.com>,
	David Hildenbrand <david@redhat.com>,
	Sven Schnelle <svens@linux.ibm.com>,
	Michal Hocko <mhocko@suse.com>, Peter Xu <peterx@redhat.com>,
	Naoya Horiguchi <naoya.horiguchi@linux.dev>,
	"Aneesh Kumar K . V" <aneesh.kumar@linux.vnet.ibm.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Prakash Sangappa <prakash.sangappa@oracle.com>,
	James Houghton <jthoughton@google.com>,
	Mina Almasry <almasrymina@google.com>,
	Pasha Tatashin <pasha.tatashin@soleen.com>,
	Axel Rasmussen <axelrasmussen@google.com>,
	Ray Fucillo <Ray.Fucillo@intersystems.com>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v2 9/9] hugetlb: clean up code checking for fault/truncation races
Date: Thu, 29 Sep 2022 14:25:16 +0800	[thread overview]
Message-ID: <cb068983-70d1-53a2-4175-e8ca7818126c@huawei.com> (raw)
In-Reply-To: <20220914221810.95771-10-mike.kravetz@oracle.com>

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