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    Subject[PATCH 5.13 037/800] teach copy_page_to_iter() to handle compound pages
    Date
    From: Al Viro <viro@zeniv.linux.org.uk>

    commit 08aa64796016cb47b2ef3d0924653b4d944b0d65 upstream.

    In situation when copy_page_to_iter() got a compound page the current
    code would only work on systems with no CONFIG_HIGHMEM. It *is* the majority
    of real-world setups, or we would've drown in bug reports by now. Still needs
    fixing.

    Current variant works for solitary page; rename that to
    __copy_page_to_iter() and turn the handling of compound pages into a loop over
    subpages.

    Cc: stable@vger.kernel.org
    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    lib/iov_iter.c | 28 +++++++++++++++++++++++++---
    1 file changed, 25 insertions(+), 3 deletions(-)

    --- a/lib/iov_iter.c
    +++ b/lib/iov_iter.c
    @@ -957,11 +957,9 @@ static inline bool page_copy_sane(struct
    return false;
    }

    -size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
    +static size_t __copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
    struct iov_iter *i)
    {
    - if (unlikely(!page_copy_sane(page, offset, bytes)))
    - return 0;
    if (i->type & (ITER_BVEC | ITER_KVEC | ITER_XARRAY)) {
    void *kaddr = kmap_atomic(page);
    size_t wanted = copy_to_iter(kaddr + offset, bytes, i);
    @@ -977,6 +975,30 @@ size_t copy_page_to_iter(struct page *pa
    else
    return copy_page_to_iter_pipe(page, offset, bytes, i);
    }
    +
    +size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
    + struct iov_iter *i)
    +{
    + size_t res = 0;
    + if (unlikely(!page_copy_sane(page, offset, bytes)))
    + return 0;
    + page += offset / PAGE_SIZE; // first subpage
    + offset %= PAGE_SIZE;
    + while (1) {
    + size_t n = __copy_page_to_iter(page, offset,
    + min(bytes, (size_t)PAGE_SIZE - offset), i);
    + res += n;
    + bytes -= n;
    + if (!bytes || !n)
    + break;
    + offset += n;
    + if (offset == PAGE_SIZE) {
    + page++;
    + offset = 0;
    + }
    + }
    + return res;
    +}
    EXPORT_SYMBOL(copy_page_to_iter);

    size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,

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