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

Luis Chamberlain
2024-06-13T07:57:27Z

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
...
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