linux-kernel - Re: [PATCH v6 16/26] huge_memory: Add vmf_insert_folio

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Message-ID: <67869d3878ee0_20fa294ae@dwillia2-xfh.jf.intel.com.notmuch>
Date: Tue, 14 Jan 2025 09:22:00 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: David Hildenbrand <david@...hat.com>, Alistair Popple
	<apopple@...dia.com>, <akpm@...ux-foundation.org>,
	<dan.j.williams@...el.com>, <linux-mm@...ck.org>
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	<zhang.lyra@...il.com>, <gerald.schaefer@...ux.ibm.com>,
	<vishal.l.verma@...el.com>, <dave.jiang@...el.com>, <logang@...tatee.com>,
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Subject: Re: [PATCH v6 16/26] huge_memory: Add vmf_insert_folio_pmd()

David Hildenbrand wrote:
> > +vm_fault_t vmf_insert_folio_pmd(struct vm_fault *vmf, struct folio *folio, bool write)
> > +{
> > +	struct vm_area_struct *vma = vmf->vma;
> > +	unsigned long addr = vmf->address & PMD_MASK;
> > +	struct mm_struct *mm = vma->vm_mm;
> > +	spinlock_t *ptl;
> > +	pgtable_t pgtable = NULL;
> > +
> > +	if (addr < vma->vm_start || addr >= vma->vm_end)
> > +		return VM_FAULT_SIGBUS;
> > +
> > +	if (WARN_ON_ONCE(folio_order(folio) != PMD_ORDER))
> > +		return VM_FAULT_SIGBUS;
> > +
> > +	if (arch_needs_pgtable_deposit()) {
> > +		pgtable = pte_alloc_one(vma->vm_mm);
> > +		if (!pgtable)
> > +			return VM_FAULT_OOM;
> > +	}
> 
> This is interesting and nasty at the same time (only to make ppc64 boo3s 
> with has tables happy). But it seems to be the right thing to do.
> 
> > +
> > +	ptl = pmd_lock(mm, vmf->pmd);
> > +	if (pmd_none(*vmf->pmd)) {
> > +		folio_get(folio);
> > +		folio_add_file_rmap_pmd(folio, &folio->page, vma);
> > +		add_mm_counter(mm, mm_counter_file(folio), HPAGE_PMD_NR);
> > +	}
> > +	insert_pfn_pmd(vma, addr, vmf->pmd, pfn_to_pfn_t(folio_pfn(folio)),
> > +		       vma->vm_page_prot, write, pgtable);
> > +	spin_unlock(ptl);
> > +	if (pgtable)
> > +		pte_free(mm, pgtable);
> 
> Ehm, are you unconditionally freeing the pgtable, even if consumed by 
> insert_pfn_pmd() ?
> 
> Note that setting pgtable to NULL in insert_pfn_pmd() when consumed will 
> not be visible here.
> 
> You'd have to pass a pointer to the ... pointer (&pgtable).
> 
> ... unless I am missing something, staring at the diff.

In fact I glazed over the fact that this has been commented on before
and assumed it was fixed:

http://lore.kernel.org/66f61ce4da80_964f2294fb@dwillia2-xfh.jf.intel.com.notmuch

So, yes, insert_pfn_pmd needs to take &pgtable to report back if the
allocation got consumed.

Good catch.