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

Miaohe Lin
2022-09-29T06:25:26Z

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