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userfaultfd: handle zeropage moves by UFFDIO_MOVE
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Current implementation of UFFDIO_MOVE fails to move zeropages and returns
EBUSY when it encounters one.  We can handle them by mapping a zeropage at
the destination and clearing the mapping at the source.  This is done both
for ordinary and for huge zeropages.

Link: https://lkml.kernel.org/r/20240131175618.2417291-1-surenb@google.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202401300107.U8iMAkTl-lkp@intel.com/
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Nicolas Geoffray <ngeoffray@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: ZhangPeng <zhangpeng362@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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@surenbaghdasaryan @akpm00
surenbaghdasaryan authored and akpm00 committed Feb 22, 2024
1 parent e777ae4 commit eb1521d
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Showing 2 changed files with 98 additions and 51 deletions.
105 changes: 61 additions & 44 deletions mm/huge_memory.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -2200,33 +2200,41 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
}

src_page = pmd_page(src_pmdval);
if (unlikely(!PageAnonExclusive(src_page))) {
spin_unlock(src_ptl);
return -EBUSY;
}

src_folio = page_folio(src_page);
folio_get(src_folio);
if (!is_huge_zero_pmd(src_pmdval)) {
if (unlikely(!PageAnonExclusive(src_page))) {
spin_unlock(src_ptl);
return -EBUSY;
}

src_folio = page_folio(src_page);
folio_get(src_folio);
} else
src_folio = NULL;

spin_unlock(src_ptl);

flush_cache_range(src_vma, src_addr, src_addr + HPAGE_PMD_SIZE);
mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, src_addr,
src_addr + HPAGE_PMD_SIZE);
mmu_notifier_invalidate_range_start(&range);

folio_lock(src_folio);
if (src_folio) {
folio_lock(src_folio);

/*
* split_huge_page walks the anon_vma chain without the page
* lock. Serialize against it with the anon_vma lock, the page
* lock is not enough.
*/
src_anon_vma = folio_get_anon_vma(src_folio);
if (!src_anon_vma) {
err = -EAGAIN;
goto unlock_folio;
}
anon_vma_lock_write(src_anon_vma);
/*
* split_huge_page walks the anon_vma chain without the page
* lock. Serialize against it with the anon_vma lock, the page
* lock is not enough.
*/
src_anon_vma = folio_get_anon_vma(src_folio);
if (!src_anon_vma) {
err = -EAGAIN;
goto unlock_folio;
}
anon_vma_lock_write(src_anon_vma);
} else
src_anon_vma = NULL;

dst_ptl = pmd_lockptr(mm, dst_pmd);
double_pt_lock(src_ptl, dst_ptl);
Expand All @@ -2235,45 +2243,54 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
err = -EAGAIN;
goto unlock_ptls;
}
if (folio_maybe_dma_pinned(src_folio) ||
!PageAnonExclusive(&src_folio->page)) {
err = -EBUSY;
goto unlock_ptls;
}
if (src_folio) {
if (folio_maybe_dma_pinned(src_folio) ||
!PageAnonExclusive(&src_folio->page)) {
err = -EBUSY;
goto unlock_ptls;
}

if (WARN_ON_ONCE(!folio_test_head(src_folio)) ||
WARN_ON_ONCE(!folio_test_anon(src_folio))) {
err = -EBUSY;
goto unlock_ptls;
}
if (WARN_ON_ONCE(!folio_test_head(src_folio)) ||
WARN_ON_ONCE(!folio_test_anon(src_folio))) {
err = -EBUSY;
goto unlock_ptls;
}

folio_move_anon_rmap(src_folio, dst_vma);
WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
folio_move_anon_rmap(src_folio, dst_vma);
WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));

src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
/* Folio got pinned from under us. Put it back and fail the move. */
if (folio_maybe_dma_pinned(src_folio)) {
set_pmd_at(mm, src_addr, src_pmd, src_pmdval);
err = -EBUSY;
goto unlock_ptls;
}
src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
/* Folio got pinned from under us. Put it back and fail the move. */
if (folio_maybe_dma_pinned(src_folio)) {
set_pmd_at(mm, src_addr, src_pmd, src_pmdval);
err = -EBUSY;
goto unlock_ptls;
}

_dst_pmd = mk_huge_pmd(&src_folio->page, dst_vma->vm_page_prot);
/* Follow mremap() behavior and treat the entry dirty after the move */
_dst_pmd = pmd_mkwrite(pmd_mkdirty(_dst_pmd), dst_vma);
_dst_pmd = mk_huge_pmd(&src_folio->page, dst_vma->vm_page_prot);
/* Follow mremap() behavior and treat the entry dirty after the move */
_dst_pmd = pmd_mkwrite(pmd_mkdirty(_dst_pmd), dst_vma);
} else {
src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
_dst_pmd = mk_huge_pmd(src_page, dst_vma->vm_page_prot);
}
set_pmd_at(mm, dst_addr, dst_pmd, _dst_pmd);

src_pgtable = pgtable_trans_huge_withdraw(mm, src_pmd);
pgtable_trans_huge_deposit(mm, dst_pmd, src_pgtable);
unlock_ptls:
double_pt_unlock(src_ptl, dst_ptl);
anon_vma_unlock_write(src_anon_vma);
put_anon_vma(src_anon_vma);
if (src_anon_vma) {
anon_vma_unlock_write(src_anon_vma);
put_anon_vma(src_anon_vma);
}
unlock_folio:
/* unblock rmap walks */
folio_unlock(src_folio);
if (src_folio)
folio_unlock(src_folio);
mmu_notifier_invalidate_range_end(&range);
folio_put(src_folio);
if (src_folio)
folio_put(src_folio);
return err;
}
#endif /* CONFIG_USERFAULTFD */
Expand Down
44 changes: 37 additions & 7 deletions mm/userfaultfd.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -959,6 +959,33 @@ static int move_swap_pte(struct mm_struct *mm,
return 0;
}

static int move_zeropage_pte(struct mm_struct *mm,
struct vm_area_struct *dst_vma,
struct vm_area_struct *src_vma,
unsigned long dst_addr, unsigned long src_addr,
pte_t *dst_pte, pte_t *src_pte,
pte_t orig_dst_pte, pte_t orig_src_pte,
spinlock_t *dst_ptl, spinlock_t *src_ptl)
{
pte_t zero_pte;

double_pt_lock(dst_ptl, src_ptl);
if (!pte_same(ptep_get(src_pte), orig_src_pte) ||
!pte_same(ptep_get(dst_pte), orig_dst_pte)) {
double_pt_unlock(dst_ptl, src_ptl);
return -EAGAIN;
}

zero_pte = pte_mkspecial(pfn_pte(my_zero_pfn(dst_addr),
dst_vma->vm_page_prot));
ptep_clear_flush(src_vma, src_addr, src_pte);
set_pte_at(mm, dst_addr, dst_pte, zero_pte);
double_pt_unlock(dst_ptl, src_ptl);

return 0;
}


/*
* The mmap_lock for reading is held by the caller. Just move the page
* from src_pmd to dst_pmd if possible, and return true if succeeded
Expand Down Expand Up @@ -1041,6 +1068,14 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
}

if (pte_present(orig_src_pte)) {
if (is_zero_pfn(pte_pfn(orig_src_pte))) {
err = move_zeropage_pte(mm, dst_vma, src_vma,
dst_addr, src_addr, dst_pte, src_pte,
orig_dst_pte, orig_src_pte,
dst_ptl, src_ptl);
goto out;
}

/*
* Pin and lock both source folio and anon_vma. Since we are in
* RCU read section, we can't block, so on contention have to
Expand Down Expand Up @@ -1404,19 +1439,14 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, struct mm_struct *mm,
err = -ENOENT;
break;
}
/* Avoid moving zeropages for now */
if (is_huge_zero_pmd(*src_pmd)) {
spin_unlock(ptl);
err = -EBUSY;
break;
}

/* Check if we can move the pmd without splitting it. */
if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
!pmd_none(dst_pmdval)) {
struct folio *folio = pfn_folio(pmd_pfn(*src_pmd));

if (!folio || !PageAnonExclusive(&folio->page)) {
if (!folio || (!is_huge_zero_page(&folio->page) &&
!PageAnonExclusive(&folio->page))) {
spin_unlock(ptl);
err = -EBUSY;
break;
Expand Down

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