[PATCH v4 13/13] mm/gup: Handle hugetlb in the generic follow_page_mask code

[Matthew Wilcox]

On Tue, Apr 02, 2024 at 05:26:28PM +0200, David Hildenbrand wrote:
> > The oops trigger is at mm/gup.c:778:
> > VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
> > 
> > So 2M passed ok, and its failing for 32M, which is cont-pmd. I'm guessing you're trying to iterate 2M into a cont-pmd folio and ending up with an unexpected tail page?
> 
> I assume we find the expected tail page, it's just that the check
> 
> VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
> 
> Doesn't make sense with hugetlb folios. We might have a tail page mapped in
> a cont-pmd entry. As soon as we call follow_huge_pmd() on "not the first
> cont-pmd entry", we trigger this check.
> 
> Likely this sanity check must also allow for hugetlb folios. Or we should
> just remove it completely.
> 
> In the past, we wanted to make sure that we never get tail pages of THP from
> PMD entries, because something would currently be broken (we don't support
> THP > PMD).

That was a practical limitation on my part.  We have various parts of
the MM which assume that pmd_page() returns a head page and until we
get all of those fixed, adding support for folios larger than PMD_SIZE
was only going to cause trouble for no significant wins.

I agree with you we should get rid of this assertion entirely.  We should
fix all the places which assume that pmd_page() returns a head page,
but that may take some time.

As an example, filemap_map_pmd() has:

       if (pmd_none(*vmf->pmd) && folio_test_pmd_mappable(folio)) {
                struct page *page = folio_file_page(folio, start);
                vm_fault_t ret = do_set_pmd(vmf, page);

and then do_set_pmd() has:

        if (page != &folio->page || folio_order(folio) != HPAGE_PMD_ORDER)
                return ret;

so we'd simply refuse to use a PMD to map a folio larger than PMD_SIZE.
There's a lot of work to be done to make this work generally (not to
mention figuring out how to handle mapcount for such folios ;-).

This particular case seems straightforward though.  Just remove the
assertion.