linux-kernel - Re: [PATCH v5 02/18] mm: Define __pte_leaf

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Date: Tue, 11 Jun 2024 17:08:45 +0200
From: Oscar Salvador <osalvador@...e.de>
To: Peter Xu <peterx@...hat.com>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jason Gunthorpe <jgg@...dia.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v5 02/18] mm: Define __pte_leaf_size() to also take a PMD
 entry

On Tue, Jun 11, 2024 at 10:17:30AM -0400, Peter Xu wrote:
> Oscar,
> 
> On Tue, Jun 11, 2024 at 11:34:23AM +0200, Oscar Salvador wrote:
> > Which means that they would be caught in the following code:
> > 
> >         ptl = pmd_huge_lock(pmd, vma);
> >         if (ptl) {
> > 	        - 8MB hugepages will be handled here
> >                 smaps_pmd_entry(pmd, addr, walk);
> >                 spin_unlock(ptl);
> >         }
> > 	/* pte stuff */
> > 	...
> 
> Just one quick comment: I think there's one challenge though as this is
> also not a generic "pmd leaf", but a pgtable page underneath.  I think it
> means smaps_pmd_entry() won't trivially work here, e.g., it will start to
> do this:
> 
> 	if (pmd_present(*pmd)) {
> 		page = vm_normal_page_pmd(vma, addr, *pmd);
> 
> Here vm_normal_page_pmd() will only work if pmd_leaf() satisfies its
> definition as:
> 
>  * - It should contain a huge PFN, which points to a huge page larger than
>  *   PAGE_SIZE of the platform.  The PFN format isn't important here.
> 
> But now it's a pgtable page, containing cont-ptes.  Similarly, I think most
> pmd_*() helpers will stop working there if we report it as a leaf.

Heh, I think I managed to confuse myself.
I do not why but I thought that

 static inline pte_t huge_ptep_get(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
 {
        if (ptep_is_8m_pmdp(mm, addr, ptep))
             ptep = pte_offset_kernel((pmd_t *)ptep, 0);
        return ptep_get(ptep);
 }

would return the address of the pmd for 8MB hugepages, but it will
return the address of the first pte?

Then yeah, this will not work as I thought.

The problem is that we do not have spare bits for 8xx to mark these ptes
as cont-ptes or mark them pte as 8MB, so I do not see a clear path on how
we could remove huge_ptep_get for 8xx.

I am really curious though how we handle that for THP? Or THP on 8xx
does not support that size?
 

-- 
Oscar Salvador
SUSE Labs