Re: [PATCH v7 06/11] filemap: cap PTE range to be created to allowed zero fill in folio_map_range()

David Hildenbrand
2024-06-13T08:07:37Z

On 13.06.24 09:57, Luis Chamberlain wrote:
> On Wed, Jun 12, 2024 at 08:08:15PM +0100, Matthew Wilcox wrote:
>> On Fri, Jun 07, 2024 at 02:58:57PM +0000, Pankaj Raghav (Samsung) wrote:
>>> From: Pankaj Raghav <p.raghav@samsung.com>
>>>
>>> Usually the page cache does not extend beyond the size of the inode,
>>> therefore, no PTEs are created for folios that extend beyond the size.
>>>
>>> But with LBS support, we might extend page cache beyond the size of the
>>> inode as we need to guarantee folios of minimum order. Cap the PTE range
>>> to be created for the page cache up to the max allowed zero-fill file
>>> end, which is aligned to the PAGE_SIZE.
>>
>> I think this is slightly misleading because we might well zero-fill
>> to the end of the folio. The issue is that we're supposed to SIGBUS
>> if userspace accesses pages which lie entirely beyond the end of this
>> file. Can you rephrase this?
>>
>> (from mmap(2))
>> SIGBUS Attempted access to a page of the buffer that lies beyond the end
>> of the mapped file. For an explanation of the treatment of the
>> bytes in the page that corresponds to the end of a mapped file
>> that is not a multiple of the page size, see NOTES.
>>
>>
>> The code is good though.
>>
>> Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
>
> Since I've been curating the respective fstests test to test for this
> POSIX corner case [0] I wanted to enable the test for tmpfs instead of
> skipping it as I originally had it, and that meant also realizing mmap(2)
> specifically says this now:
>
> Huge page (Huge TLB) mappings

Confusion alert: this likely talks about hugetlb (MAP_HUGETLB), not THP
and friends.

So it might not be required for below changes.

> ...
> For mmap(), offset must be a multiple of the underlying huge page
> size. The system automatically aligns length to be a multiple of
> the underlying huge page size.
>
> So do we need to adjust this patch with this:
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index ea78963f0956..9c8897ba90ff 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3617,6 +3617,7 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
> vm_fault_t ret = 0;
> unsigned long rss = 0;
> unsigned int nr_pages = 0, mmap_miss = 0, mmap_miss_saved, folio_type;
> + unsigned int align = PAGE_SIZE;
>
> rcu_read_lock();
> folio = next_uptodate_folio(&xas, mapping, end_pgoff);
> @@ -3636,7 +3637,10 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
> goto out;
> }
>
> - file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE) - 1;
> + if (folio_test_pmd_mappable(folio))
> + align = 1 << folio_order(folio);
> +
> + file_end = DIV_ROUND_UP(i_size_read(mapping->host), align) - 1;
> if (end_pgoff > file_end)
> end_pgoff = file_end;
>
> [0] https://lore.kernel.org/all/20240611030203.1719072-3-mcgrof@kernel.org/
>
> Luis
>

--
Cheers,

David / dhildenb