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

[Mike Kravetz]

On 07/28/22 10:02, Miaohe Lin wrote:
> On 2022/7/28 3:00, Mike Kravetz wrote:
> > On 07/27/22 17:20, Miaohe Lin wrote:
> >> 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?
> >>
> > 
> > Well, the fault mutex can only guard a single hugetlb page.  The fault mutex
> > is actually an array/table of mutexes hashed by mapping address and file index.
> > So, during truncation we take take the mutex for each page as they are
> > unmapped and removed.  So, the fault mutex only synchronizes operations
> > on one specific page.  The idea with this patch is to coordinate the fault
> > code and truncate code when operating on the same page.
> > 
> > In addition, changing the file size happens early in the truncate process
> > before taking any locks/mutexes.
> 
> I wonder whether we can somewhat live with it to make code simpler. When changing the file size happens
> after checking i_size but before taking the page table lock in hugetlb_fault, the truncate code would
> remove the hugetlb page from the page cache for us after hugetlb_fault finishes if we don't roll back
> when checking i_size again under the page table lock?
> 
> In a word, if hugetlb_fault see a truncated inode, back out early. If not, let truncate code does its
> work. So we don't need to complicate the already complicated error path. Or am I miss something?
> 

Thank you! I believe your observations and suggestions are correct.

We can just let the fault code proceed after the early "idx >= size",
and let the truncation code remove the page.  This also eliminates the
need for patch 3 (hugetlbfs: move routine remove_huge_page to hugetlb.c).

I will make these changes in the next version.
-- 
Mike Kravetz