Re: [PATCH v2 9/9] userfaultfd/shmem: modify shmem_mcopy_atomic_pte to use install_ptes

On Mon, 12 Apr 2021, Axel Rasmussen wrote:

In a previous commit, we added the mcopy_atomic_install_ptes() helper. This helper does the job of setting up PTEs for an existing page, to map it into a given VMA. It deals with both the anon and shmem cases, as well as the shared and private cases.

In other words, shmem_mcopy_atomic_pte() duplicates a case it already handles. So, expose it, and let shmem_mcopy_atomic_pte() use it directly, to reduce code duplication.

This requires that we refactor shmem_mcopy_atomic-pte() a bit:

Instead of doing accounting (shmem_recalc_inode() et al) part-way through the PTE setup, do it beforehand. This frees up mcopy_atomic_install_ptes() from having to care about this accounting, but it does mean we need to clean it up if we get a failure afterwards (shmem_uncharge()).

We can *almost* use shmem_charge() to do this, reducing code duplication. But, it does `inode->i_mapping->nrpages++`, which would double-count since shmem_add_to_page_cache() also does this.

Signed-off-by: Axel Rasmussen axelrasmussen@google.com

include/linux/userfaultfd_k.h | 5 ++++ mm/shmem.c | 52 +++++++---------------------------- mm/userfaultfd.c | 25 ++++++++--------- 3 files changed, 27 insertions(+), 55 deletions(-)

Very nice, and it gets better.

diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h index 794d1538b8ba..3e20bfa9ef80 100644 --- a/include/linux/userfaultfd_k.h +++ b/include/linux/userfaultfd_k.h @@ -53,6 +53,11 @@ enum mcopy_atomic_mode { MCOPY_ATOMIC_CONTINUE, }; +extern int mcopy_atomic_install_ptes(struct mm_struct *dst_mm, pmd_t *dst_pmd,

mcopy_atomic_install_pte throughout as before.

• 		     struct vm_area_struct *dst_vma,
  

• 		     unsigned long dst_addr, struct page *page,
  

• 		     bool newly_allocated, bool wp_copy);
  

extern ssize_t mcopy_atomic(struct mm_struct *dst_mm, unsigned long dst_start, unsigned long src_start, unsigned long len, bool *mmap_changing, __u64 mode); diff --git a/mm/shmem.c b/mm/shmem.c index 3f48cb5e8404..9b12298405a4 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -2376,10 +2376,8 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, struct address_space *mapping = inode->i_mapping; gfp_t gfp = mapping_gfp_mask(mapping); pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);

• spinlock_t *ptl; void *page_kaddr; struct page *page;
• pte_t _dst_pte, *dst_pte; int ret; pgoff_t max_off;

@@ -2389,8 +2387,10 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, if (!*pagep) { page = shmem_alloc_page(gfp, info, pgoff);

• if (!page)
  

• 	goto out_unacct_blocks;
  

• if (!page) {
  

• 	shmem_inode_unacct_blocks(inode, 1);
  

• 	goto out;
  

• }
  

if (!zeropage) { /* COPY */ page_kaddr = kmap_atomic(page); @@ -2430,59 +2430,27 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, if (ret) goto out_release;

• _dst_pte = mk_pte(page, dst_vma->vm_page_prot);
• if (dst_vma->vm_flags & VM_WRITE)

• _dst_pte = pte_mkwrite(pte_mkdirty(_dst_pte));
  

• else {

• /*
  

•  * We don't set the pte dirty if the vma has no
  

•  * VM_WRITE permission, so mark the page dirty or it
  

•  * could be freed from under us. We could do it
  

•  * unconditionally before unlock_page(), but doing it
  

•  * only if VM_WRITE is not set is faster.
  

•  */
  

• set_page_dirty(page);
  

• }
• dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
• ret = -EFAULT;
• max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
• if (unlikely(pgoff >= max_off))

• goto out_release_unlock;
  

• ret = -EEXIST;
• if (!pte_none(*dst_pte))

• goto out_release_unlock;
  

• lru_cache_add(page);
• spin_lock_irq(&info->lock); info->alloced++; inode->i_blocks += BLOCKS_PER_PAGE; shmem_recalc_inode(inode); spin_unlock_irq(&info->lock);

• inc_mm_counter(dst_mm, mm_counter_file(page));
• page_add_file_rmap(page, false);
• set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);

• ret = mcopy_atomic_install_ptes(dst_mm, dst_pmd, dst_vma, dst_addr,

• 			page, true, false);
  

• if (ret)

• goto out_release_uncharge;
  

• /* No need to invalidate - it was non-present before */
• update_mmu_cache(dst_vma, dst_addr, dst_pte);
• pte_unmap_unlock(dst_pte, ptl);

In reviewing 4/9, I said to take out mcopy_atomic_install_pte()'s set_page_dirty(). Just call it here instead, before the unlock_page(). You have an array of choices for how to write it:

if (!(dst_vma->vm_flags & VM_WRITE)) set_page_dirty(page); or if (!(dst_vma->vm_flags & VM_WRITE)) SetPageDirty(page); or set_page_dirty(page); or SetPageDirty(page);

Personally, I'd go for the last: this function has just modified the page, so it ought to mark it dirty: without second-guessing what mcopy_atomic_install_pte() might do to make that redundant in the VM_WRITE case. set_page_dirty() or SetPageDirty()? Some years ago I tended to favour the former for its preparatory PageDirty check; nowadays (on shmem) I favour the latter, to avoid the function call indirection which became more expensive with spectre+retpoline.

unlock_page(page); ret = 0; out: return ret; -out_release_unlock:

• pte_unmap_unlock(dst_pte, ptl);
• ClearPageDirty(page);

+out_release_uncharge:

Given how 4/9 was, this did still need the ClearPageDirty() before delete_from_page_cache(), to prevent a warning. But not needed here if SPD is done just before unlock_page().

delete_from_page_cache(page);

• shmem_uncharge(inode, 1);

out_release: unlock_page(page); put_page(page); -out_unacct_blocks:

• shmem_inode_unacct_blocks(inode, 1); goto out;

} #endif /* CONFIG_USERFAULTFD */ diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 8df0438f5d6a..3f73ba0b99f0 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -51,18 +51,13 @@ struct vm_area_struct *find_dst_vma(struct mm_struct *dst_mm, /*

• Install PTEs, to map dst_addr (within dst_vma) to page.

• • This function handles MCOPY_ATOMIC_CONTINUE (which is always file-backed),
• • whether or not dst_vma is VM_SHARED. It also handles the more general
• • MCOPY_ATOMIC_NORMAL case, when dst_vma is *not* VM_SHARED (it may be file
• • backed, or not).
• • Note that MCOPY_ATOMIC_NORMAL for a VM_SHARED dst_vma is handled by
• • shmem_mcopy_atomic_pte instead.

• • This function handles both MCOPY_ATOMIC_NORMAL and _CONTINUE for both shmem
• • and anon, and for both shared and private VMAs.
  */

-static int mcopy_atomic_install_ptes(struct mm_struct *dst_mm, pmd_t *dst_pmd,

• 		     struct vm_area_struct *dst_vma,
  

• 		     unsigned long dst_addr, struct page *page,
  

• 		     bool newly_allocated, bool wp_copy)
  

+int mcopy_atomic_install_ptes(struct mm_struct *dst_mm, pmd_t *dst_pmd,

• 	      struct vm_area_struct *dst_vma,
  

• 	      unsigned long dst_addr, struct page *page,
  

• 	      bool newly_allocated, bool wp_copy)
  

{ int ret; pte_t _dst_pte, *dst_pte; @@ -116,8 +111,12 @@ static int mcopy_atomic_install_ptes(struct mm_struct *dst_mm, pmd_t *dst_pmd, else page_add_new_anon_rmap(page, dst_vma, dst_addr, false);

• if (newly_allocated)

• lru_cache_add_inactive_or_unevictable(page, dst_vma);
  

• if (newly_allocated) {

• if (vma_is_shmem(dst_vma) && vm_shared)
  

• 	lru_cache_add(page);
  

• else
  

• 	lru_cache_add_inactive_or_unevictable(page, dst_vma);
  

• }

This change is not required, you'll be glad to hear. Take a look at lru_cache_add_inactive_or_unevictable() in mm/swap.c: it's a wrapper to set PageMlocked and do associated accounting (if appropriate) before doing the lru_cache_add(page). And, strictly speaking, should have been used in shmem_mcopy_atomic_pte() all along.

I say "strictly speaking" because it would be exceptional to find a page needing Mlock there, and not an error to delay that Mlock until sometime later, before reclaim. Worth a sentence in the commit message, but not worth a "Fixes:" tag.

set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte); -- 2.31.1.295.g9ea45b61b8-goog