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

[Miaohe Lin]

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@xxxxxxxxxx>
> ---

<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. :)