linux-kernel - Re: [PATCH 0/2] Fixes for hugetlb mapcount at most 1 for shared PMDs

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Message-Id: <20230126144722.8b84116c488553c79d8623ec@linux-foundation.org>
Date:   Thu, 26 Jan 2023 14:47:22 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        James Houghton <jthoughton@...gle.com>,
        Peter Xu <peterx@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...e.com>, Yang Shi <shy828301@...il.com>,
        Vishal Moola <vishal.moola@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Muchun Song <songmuchun@...edance.com>,
        Vishal Moola (Oracle) <vishal.moola@...il.com>
Subject: Re: [PATCH 0/2] Fixes for hugetlb mapcount at most 1 for shared
 PMDs

On Thu, 26 Jan 2023 14:27:19 -0800 Mike Kravetz <mike.kravetz@...cle.com> wrote:

> Ongoing folio conversions cause context conflicts in the second patch
> when applied to mm-unstable/linux-next.  I can create separate patch(es)
> if people agree with these.

I fixed things up.  queue_folios_hugetlb() is now

static int queue_folios_hugetlb(pte_t *pte, unsigned long hmask,
			       unsigned long addr, unsigned long end,
			       struct mm_walk *walk)
{
	int ret = 0;
#ifdef CONFIG_HUGETLB_PAGE
	struct queue_pages *qp = walk->private;
	unsigned long flags = (qp->flags & MPOL_MF_VALID);
	struct folio *folio;
	spinlock_t *ptl;
	pte_t entry;

	ptl = huge_pte_lock(hstate_vma(walk->vma), walk->mm, pte);
	entry = huge_ptep_get(pte);
	if (!pte_present(entry))
		goto unlock;
	folio = pfn_folio(pte_pfn(entry));
	if (!queue_folio_required(folio, qp))
		goto unlock;

	if (flags == MPOL_MF_STRICT) {
		/*
		 * STRICT alone means only detecting misplaced folio and no
		 * need to further check other vma.
		 */
		ret = -EIO;
		goto unlock;
	}

	if (!vma_migratable(walk->vma)) {
		/*
		 * Must be STRICT with MOVE*, otherwise .test_walk() have
		 * stopped walking current vma.
		 * Detecting misplaced folio but allow migrating folios which
		 * have been queued.
		 */
		ret = 1;
		goto unlock;
	}

	/*
	 * With MPOL_MF_MOVE, we try to migrate only unshared folios. If it
	 * is shared it is likely not worth migrating.
	 *
	 * To check if the folio is shared, ideally we want to make sure
	 * every page is mapped to the same process. Doing that is very
	 * expensive, so check the estimated mapcount of the folio instead.
	 */
	if (flags & (MPOL_MF_MOVE_ALL) ||
	    (flags & MPOL_MF_MOVE && folio_estimated_mapcount(folio) == 1 &&
	     !hugetlb_pmd_shared(pte))) {
		if (isolate_hugetlb(folio, qp->pagelist) &&
			(flags & MPOL_MF_STRICT))
			/*
			 * Failed to isolate folio but allow migrating pages
			 * which have been queued.
			 */
			ret = 1;
	}
unlock:
	spin_unlock(ptl);
#else
	BUG();
#endif
	return ret;
}