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hugetlb: remove VMEMMAP_SYNCHRONIZE_RCU

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The VMEMMAP_SYNCHRONIZE_RCU flag triggered synchronize_rcu() calls to
prevent a race between HVO remapping and page_ref_add_unless().  The race
could occur when a speculative PFN walker tried to modify the refcount on
a struct page that was in the process of being remapped to a fake head.

With fake heads eliminated, page_ref_add_unless() no longer needs RCU
protection.

Remove the flag and synchronize_rcu() calls.

Link: https://lkml.kernel.org/r/20260227194302.274384-15-kas@kernel.org
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Baoquan He <bhe@redhat.com>
Cc: Christoph Lameter <cl@gentwo.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Frank van der Linden <fvdl@google.com>
Cc: Harry Yoo <harry.yoo@oracle.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: WANG Xuerui <kernel@xen0n.name>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Kiryl Shutsemau 2026-02-27 19:42:52 +00:00 committed by Andrew Morton
parent 32c440d67e
commit 01b1d0ffb6
1 changed files with 4 additions and 16 deletions
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20
mm/hugetlb_vmemmap.c
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@ -47,8 +47,6 @@ struct vmemmap_remap_walk {
#define VMEMMAP_SPLIT_NO_TLB_FLUSH BIT(0)
/* Skip the TLB flush when we remap the PTE */
#define VMEMMAP_REMAP_NO_TLB_FLUSH BIT(1)
/* synchronize_rcu() to avoid writes from page_ref_add_unless() */
#define VMEMMAP_SYNCHRONIZE_RCU BIT(2)
unsigned long flags;
};
@ -409,9 +407,6 @@ static int __hugetlb_vmemmap_restore_folio(const struct hstate *h,
if (!folio_test_hugetlb_vmemmap_optimized(folio))
return 0;
if (flags & VMEMMAP_SYNCHRONIZE_RCU)
synchronize_rcu();
vmemmap_start = (unsigned long)&folio->page;
vmemmap_end = vmemmap_start + hugetlb_vmemmap_size(h);
@ -444,7 +439,7 @@ static int __hugetlb_vmemmap_restore_folio(const struct hstate *h,
*/
int hugetlb_vmemmap_restore_folio(const struct hstate *h, struct folio *folio)
{
return __hugetlb_vmemmap_restore_folio(h, folio, VMEMMAP_SYNCHRONIZE_RCU);
return __hugetlb_vmemmap_restore_folio(h, folio, 0);
}
/**
@ -467,14 +462,11 @@ long hugetlb_vmemmap_restore_folios(const struct hstate *h,
struct folio *folio, *t_folio;
long restored = 0;
long ret = 0;
unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH | VMEMMAP_SYNCHRONIZE_RCU;
unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH;
list_for_each_entry_safe(folio, t_folio, folio_list, lru) {
if (folio_test_hugetlb_vmemmap_optimized(folio)) {
ret = __hugetlb_vmemmap_restore_folio(h, folio, flags);
/* only need to synchronize_rcu() once for each batch */
flags &= ~VMEMMAP_SYNCHRONIZE_RCU;
if (ret)
break;
restored++;
@ -554,8 +546,6 @@ static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
static_branch_inc(&hugetlb_optimize_vmemmap_key);
if (flags & VMEMMAP_SYNCHRONIZE_RCU)
synchronize_rcu();
/*
* Very Subtle
* If VMEMMAP_REMAP_NO_TLB_FLUSH is set, TLB flushing is not performed
@ -613,7 +603,7 @@ void hugetlb_vmemmap_optimize_folio(const struct hstate *h, struct folio *folio)
{
LIST_HEAD(vmemmap_pages);
__hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, VMEMMAP_SYNCHRONIZE_RCU);
__hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, 0);
free_vmemmap_page_list(&vmemmap_pages);
}
@ -641,7 +631,7 @@ static void __hugetlb_vmemmap_optimize_folios(struct hstate *h,
struct folio *folio;
int nr_to_optimize;
LIST_HEAD(vmemmap_pages);
unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH | VMEMMAP_SYNCHRONIZE_RCU;
unsigned long flags = VMEMMAP_REMAP_NO_TLB_FLUSH;
nr_to_optimize = 0;
list_for_each_entry(folio, folio_list, lru) {
@ -694,8 +684,6 @@ static void __hugetlb_vmemmap_optimize_folios(struct hstate *h,
int ret;
ret = __hugetlb_vmemmap_optimize_folio(h, folio, &vmemmap_pages, flags);
/* only need to synchronize_rcu() once for each batch */
flags &= ~VMEMMAP_SYNCHRONIZE_RCU;
/*
* Pages to be freed may have been accumulated. If we