Date: Tue, 11 Jun 2024 11:34:23 +0200
From: Oscar Salvador <osalvador@...e.de>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Jason Gunthorpe <jgg@...dia.com>, Peter Xu <peterx@...hat.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 Mon, Jun 10, 2024 at 07:54:47AM +0200, Christophe Leroy wrote:
> On powerpc 8xx, when a page is 8M size, the information is in the PMD
> entry. So allow architectures to provide __pte_leaf_size() instead of
> pte_leaf_size() and provide the PMD entry to that function.
> 
> When __pte_leaf_size() is not defined, define it as a pte_leaf_size()
> so that architectures not interested in the PMD arguments are not
> impacted.
> 
> Only define a default pte_leaf_size() when __pte_leaf_size() is not
> defined to make sure nobody adds new calls to pte_leaf_size() in the
> core.

Hi Christophe,

Now I am going to give you a hard time, so sorry in advance.
I should have raised this before, but I was not fully aware of it.

There is an ongoing effort of unifying pagewalkers [1], so hugetlb does not have
to be special-cased anymore, and the operations we do for THP on page-table basis
work for hugetlb as well.

The most special bit about this is huge_ptep_get.
huge_ptep_get() gets special handled on arm/arm64/riscv and s390.

arm64 and riscv is about cont-pmd/pte and propagate the dirty/young bits bits, so that
is fine as walkers can already understand that.
s390 is a funny one because it converts pud/pmd to pte and viceversa, because hugetlb
*works* with ptes, so before returning the pte it has to transfer all
bits from PUD/PMD level into a something that PTE level can understand.
As you can imagine, this can be gone as we already have all the
information in PUD/PMD and that is all pagewalkers need.

But we are left with the one you will introduce in patch#8.

8MB pages get mapped as cont-pte, but all the information is stored in
the PMD entries (size, dirtiness, present etc).
huge_ptep_get() will return the PMD for 8MB, and so all operations hugetlb
code performs with what huge_ptep_get returns will be performed on those PMDs.

Which brings me to this point:

I do not think __pte_leaf_size is needed. AFAICS, it should be enough to define
pmd_leaf on 8xx, and return 8MB if it is a 8MB hugepage.

   #define pmd_leaf pmd_leaf
   static inline bool pmd_leaf(pmd_t pmd)
   {
          return pmd_val(pmd) & _PMD_PAGE_8M);
   }

   and then pmd_leaf_size to return _PMD_PAGE_8M.
   
This will help because on the ongoing effort of unifying hugetlb and
getting rid of huge_ptep_get() [1], pagewalkers will stumble upon the
8mb-PMD as they do for regular PMDs.

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 */
	...

where pmd_huge_lock is:

 static inline spinlock_t *pmd_huge_lock(pmd_t *pmd, struct vm_area_struct *vma)
 {
        spinlock_t *ptl = pmd_lock(vma->vm_mm, pmd);

        if (pmd_leaf(*pmd) || pmd_devmap(*pmd))
                return ptl;
        spin_unlock(ptl);
        return NULL;
 }

So, since pmd_leaf() will return true for 8MB hugepages, we are fine,
because anyway we want to perform pagetable operations on *that* PMD and
not the ptes that are cont-mapped, which is different for e.g: 512K
hugepages, where we perform it on pte level.

So I would suggest that instead of this patch, we have one implementing pmd_leaf
and pmd_leaf_size for 8Mb hugepages on power8xx, as that takes us closer to our goal of
unifying hugetlb.

[1] https://github.com/leberus/linux/tree/hugetlb-pagewalk-v2


-- 
Oscar Salvador
SUSE Labs

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