Re: [PATCH v6 1/2] mm: migration: fix the FOLL_GET failure on following huge page

[Gerald Schaefer]

On Thu, 18 Aug 2022 13:51:49 +0200
Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx> wrote:

> On Tue, 16 Aug 2022 22:43:22 -0700
> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
> > On Wed, 17 Aug 2022 03:31:37 +0000 "Wang, Haiyue" <haiyue.wang@xxxxxxxxx> wrote:
> > 
> > > > >  		}
> > > > 
> > > > I would be better to fix this for real at those three client code sites?
> > > 
> > > Then 5.19 will break for a while to wait for the final BIG patch ?
> > 
> > If that's the proposal then your [1/2] should have had a cc:stable and
> > changelog words describing the plan for 6.0.
> > 
> > But before we do that I'd like to see at least a prototype of the final
> > fixes to s390 and hugetlb, so we can assess those as preferable for
> > backporting.  I don't think they'll be terribly intrusive or risky?
> > 
> 
> The private follow_huge_pud() for s390 is just some leftover, and the
> only reason is / was that the generic version was using pte_page()
> instead of pud_page(), which would not work for s390. See also commit
> 97534127012f ("mm/hugetlb: use pmd_page() in follow_huge_pmd()").
> 
> Since commit 3a194f3f8ad01 ("mm/hugetlb: make pud_huge() and
> follow_huge_pud() aware of non-present pud entry") made
> follow_huge_pud() behave similar to follow_huge_pmd(), in particular
> also adding pud_page(), we can now switch to the generic version.
> 
> Note that we cannot support migration / hwpoison for hugetlb or THP,
> because of different layout for PTE and PMD/PUD on s390. The generic
> swp_entry functions all require proper PTEs, which wouldn't work on
> PMD/PUD entries. In theory, at least for hugetlb, due to the "fake
> PTE" conversion logic in huge_ptep_get(), we might be able to also
> fake swp_entries, but the other problem is that we do not have enough
> free bits in the PMD/PUD, so there probably will never be migration
> support for huge pages on s390.
> 
> Anyway, that should not matter wrt to switching to the generic
> follow_huge_pud(), because is_hugetlb_entry_migration() should always
> return false, and no special change to pud_huge() check should be
> needed like on x86.

From ce0150cd6f80425c702ccdc4cd8a511c47e99b67 Mon Sep 17 00:00:00 2001
From: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
Date: Thu, 18 Aug 2022 13:19:23 +0200
Subject: [PATCH] s390/hugetlb: switch to generic version of follow_huge_pud()

When pud-sized hugepages were introduced for s390, the generic version
of follow_huge_pud() was using pte_page() instead of pud_page(). This
would be wrong for s390, see also commit 97534127012f ("mm/hugetlb: use
pmd_page() in follow_huge_pmd()"). Therefore, and probably because not
all archs were supporting pud_page() at that time, a private version of
follow_huge_pud() was added for s390, correctly using pud_page().

Since commit 3a194f3f8ad01 ("mm/hugetlb: make pud_huge() and
follow_huge_pud() aware of non-present pud entry"), the generic version
of follow_huge_pud() is now also using pud_page(), and in general
behaves similar to follow_huge_pmd().

Therefore we can now switch to the generic version and get rid of the
s390-specific follow_huge_pud().

Signed-off-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
---
 arch/s390/mm/hugetlbpage.c | 10 ----------
 1 file changed, 10 deletions(-)

diff --git a/arch/s390/mm/hugetlbpage.c b/arch/s390/mm/hugetlbpage.c
index 10e51ef9c79a..c299a18273ff 100644
--- a/arch/s390/mm/hugetlbpage.c
+++ b/arch/s390/mm/hugetlbpage.c
@@ -237,16 +237,6 @@ int pud_huge(pud_t pud)
 	return pud_large(pud);
 }
 
-struct page *
-follow_huge_pud(struct mm_struct *mm, unsigned long address,
-		pud_t *pud, int flags)
-{
-	if (flags & FOLL_GET)
-		return NULL;
-
-	return pud_page(*pud) + ((address & ~PUD_MASK) >> PAGE_SHIFT);
-}
-
 bool __init arch_hugetlb_valid_size(unsigned long size)
 {
 	if (MACHINE_HAS_EDAT1 && size == PMD_SIZE)
-- 
2.34.1