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Message-Id: <20250507060256.78278-3-dev.jain@arm.com>
Date: Wed, 7 May 2025 11:32:56 +0530
From: Dev Jain <dev.jain@....com>
To: akpm@...ux-foundation.org
Cc: Liam.Howlett@...cle.com,
lorenzo.stoakes@...cle.com,
vbabka@...e.cz,
jannh@...gle.com,
pfalcato@...e.de,
linux-mm@...ck.org,
linux-kernel@...r.kernel.org,
david@...hat.com,
peterx@...hat.com,
ryan.roberts@....com,
mingo@...nel.org,
libang.li@...group.com,
maobibo@...ngson.cn,
zhengqi.arch@...edance.com,
baohua@...nel.org,
anshuman.khandual@....com,
willy@...radead.org,
ioworker0@...il.com,
yang@...amperecomputing.com,
baolin.wang@...ux.alibaba.com,
ziy@...dia.com,
hughd@...gle.com,
Dev Jain <dev.jain@....com>
Subject: [PATCH v2 2/2] mm: Optimize mremap() by PTE batching
To use PTE batching, we want to determine whether the folio mapped by
the PTE is large, thus requiring the use of vm_normal_folio(). We want
to avoid the cost of vm_normal_folio() if the code path doesn't already
require the folio. For arm64, pte_batch_hint() does the job. To generalize
this hint, add a helper which will determine whether two consecutive PTEs
point to consecutive PFNs, in which case there is a high probability that
the underlying folio is large.
Next, use folio_pte_batch() to optimize move_ptes(). On arm64, if the ptes
are painted with the contig bit, then ptep_get() will iterate through all 16
entries to collect a/d bits. Hence this optimization will result in a 16x
reduction in the number of ptep_get() calls. Next, ptep_get_and_clear()
will eventually call contpte_try_unfold() on every contig block, thus
flushing the TLB for the complete large folio range. Instead, use
get_and_clear_full_ptes() so as to elide TLBIs on each contig block, and only
do them on the starting and ending contig block.
Signed-off-by: Dev Jain <dev.jain@....com>
---
include/linux/pgtable.h | 29 +++++++++++++++++++++++++++++
mm/mremap.c | 37 ++++++++++++++++++++++++++++++-------
2 files changed, 59 insertions(+), 7 deletions(-)
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index b50447ef1c92..38dab1f562ed 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -369,6 +369,35 @@ static inline pgd_t pgdp_get(pgd_t *pgdp)
}
#endif
+/**
+ * maybe_contiguous_pte_pfns - Hint whether the page mapped by the pte belongs
+ * to a large folio.
+ * @ptep: Pointer to the page table entry.
+ * @pte: The page table entry.
+ *
+ * This helper is invoked when the caller wants to batch over a set of ptes
+ * mapping a large folio, but the concerned code path does not already have
+ * the folio. We want to avoid the cost of vm_normal_folio() only to find that
+ * the underlying folio was small; i.e keep the small folio case as fast as
+ * possible.
+ *
+ * The caller must ensure that ptep + 1 exists.
+ */
+static inline bool maybe_contiguous_pte_pfns(pte_t *ptep, pte_t pte)
+{
+ pte_t *next_ptep, next_pte;
+
+ if (pte_batch_hint(ptep, pte) != 1)
+ return true;
+
+ next_ptep = ptep + 1;
+ next_pte = ptep_get(next_ptep);
+ if (!pte_present(next_pte))
+ return false;
+
+ return unlikely(pte_pfn(next_pte) - pte_pfn(pte) == 1);
+}
+
#ifndef __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG
static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
unsigned long address,
diff --git a/mm/mremap.c b/mm/mremap.c
index 0163e02e5aa8..9c88a276bec4 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -170,6 +170,23 @@ static pte_t move_soft_dirty_pte(pte_t pte)
return pte;
}
+/* mremap a batch of PTEs mapping the same large folio */
+static int mremap_folio_pte_batch(struct vm_area_struct *vma, unsigned long addr,
+ pte_t *ptep, pte_t pte, int max_nr)
+{
+ const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
+ struct folio *folio;
+ int nr = 1;
+
+ if ((max_nr != 1) && maybe_contiguous_pte_pfns(ptep, pte)) {
+ folio = vm_normal_folio(vma, addr, pte);
+ if (folio && folio_test_large(folio))
+ nr = folio_pte_batch(folio, addr, ptep, pte, max_nr,
+ flags, NULL, NULL, NULL);
+ }
+ return nr;
+}
+
static int move_ptes(struct pagetable_move_control *pmc,
unsigned long extent, pmd_t *old_pmd, pmd_t *new_pmd)
{
@@ -177,7 +194,7 @@ static int move_ptes(struct pagetable_move_control *pmc,
bool need_clear_uffd_wp = vma_has_uffd_without_event_remap(vma);
struct mm_struct *mm = vma->vm_mm;
pte_t *old_ptep, *new_ptep;
- pte_t pte;
+ pte_t old_pte, pte;
pmd_t dummy_pmdval;
spinlock_t *old_ptl, *new_ptl;
bool force_flush = false;
@@ -186,6 +203,7 @@ static int move_ptes(struct pagetable_move_control *pmc,
unsigned long old_end = old_addr + extent;
unsigned long len = old_end - old_addr;
int err = 0;
+ int max_nr;
/*
* When need_rmap_locks is true, we take the i_mmap_rwsem and anon_vma
@@ -236,12 +254,13 @@ static int move_ptes(struct pagetable_move_control *pmc,
flush_tlb_batched_pending(vma->vm_mm);
arch_enter_lazy_mmu_mode();
- for (; old_addr < old_end; old_ptep++, old_addr += PAGE_SIZE,
- new_ptep++, new_addr += PAGE_SIZE) {
- if (pte_none(ptep_get(old_ptep)))
+ for (int nr = 1; old_addr < old_end; old_ptep += nr, old_addr += nr * PAGE_SIZE,
+ new_ptep += nr, new_addr += nr * PAGE_SIZE) {
+ max_nr = (old_end - old_addr) >> PAGE_SHIFT;
+ old_pte = ptep_get(old_ptep);
+ if (pte_none(old_pte))
continue;
- pte = ptep_get_and_clear(mm, old_addr, old_ptep);
/*
* If we are remapping a valid PTE, make sure
* to flush TLB before we drop the PTL for the
@@ -253,8 +272,12 @@ static int move_ptes(struct pagetable_move_control *pmc,
* the TLB entry for the old mapping has been
* flushed.
*/
- if (pte_present(pte))
+ if (pte_present(old_pte)) {
+ nr = mremap_folio_pte_batch(vma, old_addr, old_ptep,
+ old_pte, max_nr);
force_flush = true;
+ }
+ pte = get_and_clear_full_ptes(mm, old_addr, old_ptep, nr, 0);
pte = move_pte(pte, old_addr, new_addr);
pte = move_soft_dirty_pte(pte);
@@ -267,7 +290,7 @@ static int move_ptes(struct pagetable_move_control *pmc,
else if (is_swap_pte(pte))
pte = pte_swp_clear_uffd_wp(pte);
}
- set_pte_at(mm, new_addr, new_ptep, pte);
+ set_ptes(mm, new_addr, new_ptep, pte, nr);
}
}
--
2.30.2
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