Re: [PATCH v2 2/9] userfaultfd/shmem: combine shmem_{mcopy_atomic,mfill_zeropage}_pte

From: Hugh Dickins
Date: Wed Apr 14 2021 - 02:51:35 EST

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In reply to: Axel Rasmussen: "[PATCH v2 2/9] userfaultfd/shmem: combine shmem_{mcopy_atomic,mfill_zeropage}_pte"
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On Mon, 12 Apr 2021, Axel Rasmussen wrote:

> Previously, we did a dance where we had one calling path in
> userfaultfd.c (mfill_atomic_pte), but then we split it into two in
> shmem_fs.h (shmem_{mcopy_atomic,mfill_zeropage}_pte), and then rejoined
> into a single shared function in shmem.c (shmem_mfill_atomic_pte).
>
> This is all a bit overly complex. Just call the single combined shmem
> function directly, allowing us to clean up various branches,
> boilerplate, etc.
>
> While we're touching this function, two other small cleanup changes:
> - offset is equivalent to pgoff, so we can get rid of offset entirely.
> - Split two VM_BUG_ON cases into two statements. This means the line
> number reported when the BUG is hit specifies exactly which condition
> was true.
>
> Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>
> Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>

Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
though you've dropped one minor fix I did like, see below...

> ---
> include/linux/shmem_fs.h | 15 +++++-------
> mm/shmem.c | 52 +++++++++++++---------------------------
> mm/userfaultfd.c | 10 +++-----
> 3 files changed, 25 insertions(+), 52 deletions(-)
>
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index d82b6f396588..919e36671fe6 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -122,21 +122,18 @@ static inline bool shmem_file(struct file *file)
> extern bool shmem_charge(struct inode *inode, long pages);
> extern void shmem_uncharge(struct inode *inode, long pages);
>
> +#ifdef CONFIG_USERFAULTFD
> #ifdef CONFIG_SHMEM
> extern int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> struct vm_area_struct *dst_vma,
> unsigned long dst_addr,
> unsigned long src_addr,
> + bool zeropage,
> struct page **pagep);
> -extern int shmem_mfill_zeropage_pte(struct mm_struct *dst_mm,
> - pmd_t *dst_pmd,
> - struct vm_area_struct *dst_vma,
> - unsigned long dst_addr);
> -#else
> +#else /* !CONFIG_SHMEM */
> #define shmem_mcopy_atomic_pte(dst_mm, dst_pte, dst_vma, dst_addr, \

In a previous version, you quietly corrected that "dst_pte" to "dst_pmd":
of course it makes no difference to the code generated, but it was a good
correction, helping to prevent confusion.

> - src_addr, pagep) ({ BUG(); 0; })
> -#define shmem_mfill_zeropage_pte(dst_mm, dst_pmd, dst_vma, \
> - dst_addr) ({ BUG(); 0; })
> -#endif
> + src_addr, zeropage, pagep) ({ BUG(); 0; })
> +#endif /* CONFIG_SHMEM */
> +#endif /* CONFIG_USERFAULTFD */
>
> #endif
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 26c76b13ad23..b72c55aa07fc 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2354,13 +2354,14 @@ static struct inode *shmem_get_inode(struct super_block *sb, const struct inode
> return inode;
> }
>
> -static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
> - pmd_t *dst_pmd,
> - struct vm_area_struct *dst_vma,
> - unsigned long dst_addr,
> - unsigned long src_addr,
> - bool zeropage,
> - struct page **pagep)
> +#ifdef CONFIG_USERFAULTFD
> +int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm,
> + pmd_t *dst_pmd,
> + struct vm_area_struct *dst_vma,
> + unsigned long dst_addr,
> + unsigned long src_addr,
> + bool zeropage,
> + struct page **pagep)
> {
> struct inode *inode = file_inode(dst_vma->vm_file);
> struct shmem_inode_info *info = SHMEM_I(inode);
> @@ -2372,7 +2373,7 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
> struct page *page;
> pte_t _dst_pte, *dst_pte;
> int ret;
> - pgoff_t offset, max_off;
> + pgoff_t max_off;
>
> ret = -ENOMEM;
> if (!shmem_inode_acct_block(inode, 1))
> @@ -2383,7 +2384,7 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
> if (!page)
> goto out_unacct_blocks;
>
> - if (!zeropage) { /* mcopy_atomic */
> + if (!zeropage) { /* COPY */
> page_kaddr = kmap_atomic(page);
> ret = copy_from_user(page_kaddr,
> (const void __user *)src_addr,
> @@ -2397,7 +2398,7 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
> /* don't free the page */
> return -ENOENT;
> }
> - } else { /* mfill_zeropage_atomic */
> + } else { /* ZEROPAGE */
> clear_highpage(page);
> }
> } else {
> @@ -2405,15 +2406,15 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
> *pagep = NULL;
> }
>
> - VM_BUG_ON(PageLocked(page) || PageSwapBacked(page));
> + VM_BUG_ON(PageLocked(page));
> + VM_BUG_ON(PageSwapBacked(page));
> __SetPageLocked(page);
> __SetPageSwapBacked(page);
> __SetPageUptodate(page);
>
> ret = -EFAULT;
> - offset = linear_page_index(dst_vma, dst_addr);
> max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> - if (unlikely(offset >= max_off))
> + if (unlikely(pgoff >= max_off))
> goto out_release;
>
> ret = shmem_add_to_page_cache(page, mapping, pgoff, NULL,
> @@ -2439,7 +2440,7 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
>
> ret = -EFAULT;
> max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> - if (unlikely(offset >= max_off))
> + if (unlikely(pgoff >= max_off))
> goto out_release_unlock;
>
> ret = -EEXIST;
> @@ -2476,28 +2477,7 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
> shmem_inode_unacct_blocks(inode, 1);
> goto out;
> }
> -
> -int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm,
> - pmd_t *dst_pmd,
> - struct vm_area_struct *dst_vma,
> - unsigned long dst_addr,
> - unsigned long src_addr,
> - struct page **pagep)
> -{
> - return shmem_mfill_atomic_pte(dst_mm, dst_pmd, dst_vma,
> - dst_addr, src_addr, false, pagep);
> -}
> -
> -int shmem_mfill_zeropage_pte(struct mm_struct *dst_mm,
> - pmd_t *dst_pmd,
> - struct vm_area_struct *dst_vma,
> - unsigned long dst_addr)
> -{
> - struct page *page = NULL;
> -
> - return shmem_mfill_atomic_pte(dst_mm, dst_pmd, dst_vma,
> - dst_addr, 0, true, &page);
> -}
> +#endif /* CONFIG_USERFAULTFD */
>
> #ifdef CONFIG_TMPFS
> static const struct inode_operations shmem_symlink_inode_operations;
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index e14b3820c6a8..23fa2583bbd1 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -440,13 +440,9 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
> dst_vma, dst_addr);
> } else {
> VM_WARN_ON_ONCE(wp_copy);
> - if (!zeropage)
> - err = shmem_mcopy_atomic_pte(dst_mm, dst_pmd,
> - dst_vma, dst_addr,
> - src_addr, page);
> - else
> - err = shmem_mfill_zeropage_pte(dst_mm, dst_pmd,
> - dst_vma, dst_addr);
> + err = shmem_mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
> + dst_addr, src_addr, zeropage,
> + page);
> }
>
> return err;
> --
> 2.31.1.295.g9ea45b61b8-goog
>
>

Next message: Keqian Zhu: "[PATCH v3 0/2] kvm/arm64: Try stage2 block mapping for host device MMIO"
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In reply to: Axel Rasmussen: "[PATCH v2 2/9] userfaultfd/shmem: combine shmem_{mcopy_atomic,mfill_zeropage}_pte"
Next in thread: Axel Rasmussen: "[PATCH v2 4/9] userfaultfd/shmem: support UFFDIO_CONTINUE for shmem"
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