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Subject[PATCH 5.4 50/71] mm/thp: make is_huge_zero_pmd() safe and quicker
Date
From: Hugh Dickins <hughd@google.com>

commit 3b77e8c8cde581dadab9a0f1543a347e24315f11 upstream.

Most callers of is_huge_zero_pmd() supply a pmd already verified
present; but a few (notably zap_huge_pmd()) do not - it might be a pmd
migration entry, in which the pfn is encoded differently from a present
pmd: which might pass the is_huge_zero_pmd() test (though not on x86,
since L1TF forced us to protect against that); or perhaps even crash in
pmd_page() applied to a swap-like entry.

Make it safe by adding pmd_present() check into is_huge_zero_pmd()
itself; and make it quicker by saving huge_zero_pfn, so that
is_huge_zero_pmd() will not need to do that pmd_page() lookup each time.

__split_huge_pmd_locked() checked pmd_trans_huge() before: that worked,
but is unnecessary now that is_huge_zero_pmd() checks present.

Link: https://lkml.kernel.org/r/21ea9ca-a1f5-8b90-5e88-95fb1c49bbfa@google.com
Fixes: e71769ae5260 ("mm: enable thp migration for shmem thp")
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jue Wang <juew@google.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Wang Yugui <wangyugui@e16-tech.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/linux/huge_mm.h | 8 +++++++-
mm/huge_memory.c | 5 ++++-
2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index d8b86fd39113..d2dbe462efee 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -259,6 +259,7 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr,
extern vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf, pmd_t orig_pmd);

extern struct page *huge_zero_page;
+extern unsigned long huge_zero_pfn;

static inline bool is_huge_zero_page(struct page *page)
{
@@ -267,7 +268,7 @@ static inline bool is_huge_zero_page(struct page *page)

static inline bool is_huge_zero_pmd(pmd_t pmd)
{
- return is_huge_zero_page(pmd_page(pmd));
+ return READ_ONCE(huge_zero_pfn) == pmd_pfn(pmd) && pmd_present(pmd);
}

static inline bool is_huge_zero_pud(pud_t pud)
@@ -398,6 +399,11 @@ static inline bool is_huge_zero_page(struct page *page)
return false;
}

+static inline bool is_huge_zero_pmd(pmd_t pmd)
+{
+ return false;
+}
+
static inline bool is_huge_zero_pud(pud_t pud)
{
return false;
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e74c5a505e2b..47d95048f31e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -61,6 +61,7 @@ static struct shrinker deferred_split_shrinker;

static atomic_t huge_zero_refcount;
struct page *huge_zero_page __read_mostly;
+unsigned long huge_zero_pfn __read_mostly = ~0UL;

bool transparent_hugepage_enabled(struct vm_area_struct *vma)
{
@@ -97,6 +98,7 @@ static struct page *get_huge_zero_page(void)
__free_pages(zero_page, compound_order(zero_page));
goto retry;
}
+ WRITE_ONCE(huge_zero_pfn, page_to_pfn(zero_page));

/* We take additional reference here. It will be put back by shrinker */
atomic_set(&huge_zero_refcount, 2);
@@ -146,6 +148,7 @@ static unsigned long shrink_huge_zero_page_scan(struct shrinker *shrink,
if (atomic_cmpxchg(&huge_zero_refcount, 1, 0) == 1) {
struct page *zero_page = xchg(&huge_zero_page, NULL);
BUG_ON(zero_page == NULL);
+ WRITE_ONCE(huge_zero_pfn, ~0UL);
__free_pages(zero_page, compound_order(zero_page));
return HPAGE_PMD_NR;
}
@@ -2182,7 +2185,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
return;
}

- if (pmd_trans_huge(*pmd) && is_huge_zero_pmd(*pmd)) {
+ if (is_huge_zero_pmd(*pmd)) {
/*
* FIXME: Do we want to invalidate secondary mmu by calling
* mmu_notifier_invalidate_range() see comments below inside
--
2.30.2
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