linux-kernel - Re: [PATCH v3 06/22] mm: Always use page table accessor functions

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Message-ID: <0019d675-ce3d-4a5c-89ed-f126c45145c9@kernel.org>
Date: Wed, 26 Nov 2025 12:08:19 +0100
From: "Christophe Leroy (CS GROUP)" <chleroy@...nel.org>
To: Samuel Holland <samuel.holland@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Paul Walmsley <pjw@...nel.org>,
 linux-riscv@...ts.infradead.org, Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>, linux-mm@...ck.org
Cc: devicetree@...r.kernel.org, Suren Baghdasaryan <surenb@...gle.com>,
 linux-kernel@...r.kernel.org, Mike Rapoport <rppt@...nel.org>,
 Michal Hocko <mhocko@...e.com>, Conor Dooley <conor@...nel.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Alexandre Ghiti <alex@...ti.fr>,
 Emil Renner Berthing <kernel@...il.dk>, Rob Herring <robh+dt@...nel.org>,
 Vlastimil Babka <vbabka@...e.cz>, "Liam R . Howlett"
 <Liam.Howlett@...cle.com>, Julia Lawall <Julia.Lawall@...ia.fr>,
 Nicolas Palix <nicolas.palix@...g.fr>
Subject: Re: [PATCH v3 06/22] mm: Always use page table accessor functions



Le 13/11/2025 à 02:45, Samuel Holland a écrit :
> Some platforms need to fix up the values when reading or writing page
> tables. Because of this, the accessors must always be used; it is not
> valid to simply dereference a pXX_t pointer.
> 
> Fix all of the instances of this pattern in generic code, mostly by
> applying the below coccinelle semantic patch, repeated for each page
> table level. Some additional fixes were applied manually, mostly to
> macros where type information is unavailable.
> 
> In a few places, a `pte_t *` or `pmd_t *` is actually a pointer to a PTE
> or PMDE value stored on the stack, not a pointer to a page table. In
> those cases, it is not appropriate to use the accessors, because the
> value is not globally visible, and any transformation from pXXp_get()
> has already been applied. Those places are marked by naming the pointer
> `ptentp` or `pmdvalp`, as opposed to `ptep` or `pmdp`.
> 
...

> 
> Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
> ---
> This commit covers some of the same changes as an existing series from
> Anshuman Khandual[1]. Unlike that series, this commit is a purely
> mechanical conversion to demonstrate the RISC-V changes, so it does not
> insert local variables to avoid redundant calls to the accessors. A
> manual conversion like in that series could improve performance.

And this commit has the same problem as the series from Anshuman, see [2]:

Before the patch, as an exemple on powerpc/32 mm_find_pmd() was:

00001860 <mm_find_pmd>:
     1860:       80 63 00 18     lwz     r3,24(r3)
     1864:       54 84 65 3a     rlwinm  r4,r4,12,20,29
     1868:       7c 63 22 14     add     r3,r3,r4
     186c:       4e 80 00 20     blr

Now it is:

00001860 <mm_find_pmd>:
     1860:       81 23 00 18     lwz     r9,24(r3)
     1864:       54 84 65 3a     rlwinm  r4,r4,12,20,29
     1868:       7d 49 20 2e     lwzx    r10,r9,r4  <== Pointless read
     186c:       7c 69 22 14     add     r3,r9,r4
     1870:       7d 49 20 2e     lwzx    r10,r9,r4  <== Pointless read
     1874:       7d 29 20 2e     lwzx    r9,r9,r4   <== Pointless read
     1878:       4e 80 00 20     blr

To avoid it you need something like:

diff --git a/mm/rmap.c b/mm/rmap.c
index aafefc1d7955..0c61153f4ea9 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -819,15 +819,15 @@ pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned 
long address)
  	pmd_t *pmd = NULL;

  	pgd = pgd_offset(mm, address);
-	if (!pgd_present(pgdp_get(pgd)))
+	if (!mm_p4d_folded(mm) && !pgd_present(pgdp_get(pgd)))
  		goto out;

  	p4d = p4d_offset(pgd, address);
-	if (!p4d_present(p4dp_get(p4d)))
+	if (!mm_pud_folded(mm) && !p4d_present(p4dp_get(p4d)))
  		goto out;

  	pud = pud_offset(p4d, address);
-	if (!pud_present(pudp_get(pud)))
+	if (!mm_pmd_folded(mm) && !pud_present(pudp_get(pud)))
  		goto out;

  	pmd = pmd_offset(pud, address);


[2] 
https://lore.kernel.org/linux-mm/f40ea8bf-0862-41a7-af19-70bfbd838568@csgroup.eu/

> 
> [1]: https://lore.kernel.org/linux-mm/20240917073117.1531207-1-anshuman.khandual@arm.com/
> 

Christophe