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Date: Wed, 21 Feb 2024 19:49:22 +0800
From: Peter Xu <peterx@redhat.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	James Houghton <jthoughton@google.com>,
	David Hildenbrand <david@redhat.com>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	Yang Shi <shy828301@gmail.com>, linux-riscv@lists.infradead.org,
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	"Aneesh Kumar K . V" <aneesh.kumar@kernel.org>,
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	Mike Kravetz <mike.kravetz@oracle.com>,
	Muchun Song <muchun.song@linux.dev>,
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	Christoph Hellwig <hch@infradead.org>,
	Lorenzo Stoakes <lstoakes@gmail.com>,
	Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v2 10/13] mm/gup: Handle huge pud for follow_pud_mask()
Message-ID: <ZdXjQjow4fa41ORD@x1n>
References: <20240103091423.400294-1-peterx@redhat.com>
 <20240103091423.400294-11-peterx@redhat.com>
 <20240115184900.GV734935@nvidia.com>
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In-Reply-To: <20240115184900.GV734935@nvidia.com>

On Mon, Jan 15, 2024 at 02:49:00PM -0400, Jason Gunthorpe wrote:
> On Wed, Jan 03, 2024 at 05:14:20PM +0800, peterx@redhat.com wrote:
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 63845b3ec44f..760406180222 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -525,6 +525,70 @@ static struct page *no_page_table(struct vm_area_struct *vma,
> >  	return NULL;
> >  }
> >  
> > +#ifdef CONFIG_PGTABLE_HAS_HUGE_LEAVES
> > +static struct page *follow_huge_pud(struct vm_area_struct *vma,
> > +				    unsigned long addr, pud_t *pudp,
> > +				    int flags, struct follow_page_context *ctx)
> > +{
> > +	struct mm_struct *mm = vma->vm_mm;
> > +	struct page *page;
> > +	pud_t pud = *pudp;
> > +	unsigned long pfn = pud_pfn(pud);
> > +	int ret;
> > +
> > +	assert_spin_locked(pud_lockptr(mm, pudp));
> > +
> > +	if ((flags & FOLL_WRITE) && !pud_write(pud))
> > +		return NULL;
> > +
> > +	if (!pud_present(pud))
> > +		return NULL;
> > +
> > +	pfn += (addr & ~PUD_MASK) >> PAGE_SHIFT;
> > +
> > +#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> > +	if (pud_devmap(pud)) {
> 
> Can this use IS_ENABLED(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD) ?

Sure.

> 
> > +		/*
> > +		 * device mapped pages can only be returned if the caller
> > +		 * will manage the page reference count.
> > +		 *
> > +		 * At least one of FOLL_GET | FOLL_PIN must be set, so
> > +		 * assert that here:
> > +		 */
> > +		if (!(flags & (FOLL_GET | FOLL_PIN)))
> > +			return ERR_PTR(-EEXIST);
> > +
> > +		if (flags & FOLL_TOUCH)
> > +			touch_pud(vma, addr, pudp, flags & FOLL_WRITE);
> > +
> > +		ctx->pgmap = get_dev_pagemap(pfn, ctx->pgmap);
> > +		if (!ctx->pgmap)
> > +			return ERR_PTR(-EFAULT);
> > +	}
> > +#endif	/* CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD */
> > +	page = pfn_to_page(pfn);
> > +
> > +	if (!pud_devmap(pud) && !pud_write(pud) &&
> > +	    gup_must_unshare(vma, flags, page))
> > +		return ERR_PTR(-EMLINK);
> > +
> > +	ret = try_grab_page(page, flags);
> > +	if (ret)
> > +		page = ERR_PTR(ret);
> > +	else
> > +		ctx->page_mask = HPAGE_PUD_NR - 1;
> > +
> > +	return page;
> > +}
> > +#else  /* CONFIG_PGTABLE_HAS_HUGE_LEAVES */
> > +static struct page *follow_huge_pud(struct vm_area_struct *vma,
> > +				    unsigned long addr, pud_t *pudp,
> > +				    int flags, struct follow_page_context *ctx)
> > +{
> > +	return NULL;
> > +}
> > +#endif	/* CONFIG_PGTABLE_HAS_HUGE_LEAVES */
> > +
> >  static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
> >  		pte_t *pte, unsigned int flags)
> >  {
> > @@ -760,11 +824,11 @@ static struct page *follow_pud_mask(struct vm_area_struct *vma,
> >  
> >  	pudp = pud_offset(p4dp, address);
> >  	pud = READ_ONCE(*pudp);
> > -	if (pud_none(pud))
> > +	if (pud_none(pud) || !pud_present(pud))
> >  		return no_page_table(vma, flags, address);
> 
> Isn't 'pud_none() || !pud_present()' redundent? A none pud is
> non-present, by definition?

Hmm yes, seems redundant.  Let me drop it.

> 
> > -	if (pud_devmap(pud)) {
> > +	if (pud_huge(pud)) {
> >  		ptl = pud_lock(mm, pudp);
> > -		page = follow_devmap_pud(vma, address, pudp, flags, &ctx->pgmap);
> > +		page = follow_huge_pud(vma, address, pudp, flags, ctx);
> >  		spin_unlock(ptl);
> >  		if (page)
> >  			return page;
> 
> Otherwise it looks OK to me
> 
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Thanks!

-- 
Peter Xu