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mm: optimize mremap() by PTE batching

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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.

For split folios, there will be no pte batching; nr_ptes will be 1.  For
pagetable splitting, the ptes will still point to the same large folio;
for arm64, this results in the optimization described above, and for other
arches (including the general case), a minor improvement is expected due
to a reduction in the number of function calls.

Link: https://lkml.kernel.org/r/20250610035043.75448-3-dev.jain@arm.com
Signed-off-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Bang Li <libang.li@antgroup.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: bibo mao <maobibo@loongson.cn>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yang Shi <yang@os.amperecomputing.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Dev Jain 2025-06-10 09:20:43 +05:30 committed by Andrew Morton
parent 94dab12d86
commit f822a9a81a
1 changed files with 32 additions and 7 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

39
mm/mremap.c
View file

@ -170,6 +170,23 @@ static pte_t move_soft_dirty_pte(pte_t pte)
return pte;
}
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;
if (max_nr == 1)
return 1;
folio = vm_normal_folio(vma, addr, pte);
if (!folio || !folio_test_large(folio))
return 1;
return folio_pte_batch(folio, addr, ptep, pte, max_nr, flags, NULL,
NULL, NULL);
}
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;
@ -185,6 +202,8 @@ static int move_ptes(struct pagetable_move_control *pmc,
unsigned long new_addr = pmc->new_addr;
unsigned long old_end = old_addr + extent;
unsigned long len = old_end - old_addr;
int max_nr_ptes;
int nr_ptes;
int err = 0;
/*
@ -236,14 +255,16 @@ 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) {
for (; old_addr < old_end; old_ptep += nr_ptes, old_addr += nr_ptes * PAGE_SIZE,
new_ptep += nr_ptes, new_addr += nr_ptes * PAGE_SIZE) {
VM_WARN_ON_ONCE(!pte_none(*new_ptep));
if (pte_none(ptep_get(old_ptep)))
nr_ptes = 1;
max_nr_ptes = (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
@ -255,8 +276,12 @@ 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_ptes = mremap_folio_pte_batch(vma, old_addr, old_ptep,
old_pte, max_nr_ptes);
force_flush = true;
}
pte = get_and_clear_full_ptes(mm, old_addr, old_ptep, nr_ptes, 0);
pte = move_pte(pte, old_addr, new_addr);
pte = move_soft_dirty_pte(pte);
@ -269,7 +294,7 @@ 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_ptes);
}
}