linux-kernel - Re: [PATCH v4 13/13] mm/gup: Handle hugetlb in the generic follow_page

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Date: Tue, 2 Apr 2024 17:00:04 +0100
From: Matthew Wilcox <willy@...radead.org>
To: David Hildenbrand <david@...hat.com>
Cc: Ryan Roberts <ryan.roberts@....com>, peterx@...hat.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Yang Shi <shy828301@...il.com>,
	"Kirill A . Shutemov" <kirill@...temov.name>,
	Mike Kravetz <mike.kravetz@...cle.com>,
	John Hubbard <jhubbard@...dia.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Andrew Jones <andrew.jones@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>,
	linux-riscv@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>,
	Lorenzo Stoakes <lstoakes@...il.com>,
	Rik van Riel <riel@...riel.com>,
	linux-arm-kernel@...ts.infradead.org,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Aneesh Kumar K . V" <aneesh.kumar@...nel.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	James Houghton <jthoughton@...gle.com>,
	Jason Gunthorpe <jgg@...dia.com>, Mike Rapoport <rppt@...nel.org>,
	Axel Rasmussen <axelrasmussen@...gle.com>
Subject: Re: [PATCH v4 13/13] mm/gup: Handle hugetlb in the generic
 follow_page_mask code

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.