linux-kernel - Re: [PATCH RFC 06/12] mm/gup: Drop folio_fast_pin

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Message-ID: <ZV-p7haI5SmIYACs@x1n>
Date:   Thu, 23 Nov 2023 14:37:18 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        James Houghton <jthoughton@...gle.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        Yang Shi <shy828301@...il.com>,
        Rik van Riel <riel@...riel.com>,
        Hugh Dickins <hughd@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        Mike Rapoport <rppt@...nel.org>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH RFC 06/12] mm/gup: Drop folio_fast_pin_allowed() in
 hugepd processing

On Thu, Nov 23, 2023 at 06:22:33PM +0000, Christophe Leroy wrote:
> > For fast-gup I think the hugepd code is in use, however for walk_page_*
> > apis hugepd code shouldn't be reached iiuc as we have the hugetlb specific
> > handling (walk_hugetlb_range()), so anything within walk_pgd_range() to hit
> > a hugepd can be dead code to me (but note that this "dead code" is good
> > stuff to me, if one would like to merge hugetlb instead into generic mm).
> 
> Not sure what you mean here. What do you mean by "dead code" ?
> A hugepage directory can be plugged at any page level, from PGD to PMD.
> So the following bit in walk_pgd_range() is valid and not dead:
> 
> 		if (is_hugepd(__hugepd(pgd_val(*pgd))))
> 			err = walk_hugepd_range((hugepd_t *)pgd, addr, next, walk, PGDIR_SHIFT);

IMHO it boils down to the question on whether hugepd is only used in
hugetlbfs.  I think I already mentioned that above, but I can be more
explicit; what I see is that from higher stack in __walk_page_range():

	if (is_vm_hugetlb_page(vma)) {
		if (ops->hugetlb_entry)
			err = walk_hugetlb_range(start, end, walk);
	} else
		err = walk_pgd_range(start, end, walk);

It means to me as long as the vma is hugetlb, it'll not trigger any code in
walk_pgd_range(), but only walk_hugetlb_range().  Do you perhaps mean
hugepd is used outside hugetlbfs?

Thanks,

-- 
Peter Xu