Re: [RFC PATCH v4 4/8] hugetlbfs: catch and handle truncate racing with page faults

Miaohe Lin
2022-07-27T09:20:40Z

On 2022/7/7 4:23, Mike Kravetz wrote:
> Most hugetlb fault handling code checks for faults beyond i_size.
> While there are early checks in the code paths, the most difficult
> to handle are those discovered after taking the page table lock.
> At this point, we have possibly allocated a page and consumed
> associated reservations and possibly added the page to the page cache.
>
> When discovering a fault beyond i_size, be sure to:
> - Remove the page from page cache, else it will sit there until the
> file is removed.
> - Do not restore any reservation for the page consumed. Otherwise
> there will be an outstanding reservation for an offset beyond the
> end of file.
>
> The 'truncation' code in remove_inode_hugepages must deal with fault
> code potentially removing a page/folio from the cache after the page was
> returned by filemap_get_folios and before locking the page. This can be
> discovered by a change in folio_mapping() after taking folio lock. In
> addition, this code must deal with fault code potentially consuming
> and returning reservations. To synchronize this, remove_inode_hugepages
> will now take the fault mutex for ALL indices in the hole or truncated
> range. In this way, it KNOWS fault code has finished with the page/index
> OR fault code will see the updated file size.
>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---

<snip>

> @@ -5606,8 +5610,10 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
>
> ptl = huge_pte_lock(h, mm, ptep);
> size = i_size_read(mapping->host) >> huge_page_shift(h);
> - if (idx >= size)
> + if (idx >= size) {
> + beyond_i_size = true;

Thanks for your patch. There is one question:

Since races between hugetlb pagefault and truncate is guarded by hugetlb_fault_mutex,
do we really need to check it again after taking the page table lock?

BTW: I will learn more about this series when I have enough time. Thanks for your work. :)