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    SubjectRe: [RFC][PATCH] iov_iter: Add ITER_MAPPING
    Date
    Matthew Wilcox <willy@infradead.org> wrote:

    > It's perfectly legal to have compound pages in the page cache. Call
    > find_subpage(page, xas.xa_index) unconditionally.

    Like this?

    #define iterate_mapping(i, n, __v, skip, STEP) { \
    struct page *page; \
    size_t wanted = n, seg, offset; \
    loff_t start = i->mapping_start + skip; \
    pgoff_t index = start >> PAGE_SHIFT; \
    \
    XA_STATE(xas, &i->mapping->i_pages, index); \
    \
    rcu_read_lock(); \
    xas_for_each(&xas, page, ULONG_MAX) { \
    if (xas_retry(&xas, page) || xa_is_value(page)) { \
    WARN_ON(1); \
    break; \
    } \
    __v.bv_page = find_subpage(page, xas.xa_index); \
    offset = (i->mapping_start + skip) & ~PAGE_MASK; \
    seg = PAGE_SIZE - offset; \
    __v.bv_offset = offset; \
    __v.bv_len = min(n, seg); \
    (void)(STEP); \
    n -= __v.bv_len; \
    skip += __v.bv_len; \
    if (n == 0) \
    break; \
    } \
    rcu_read_unlock(); \
    n = wanted - n; \
    }

    Note that the walk is not restartable - and the array is supposed to have been
    fully populated by the caller for the range specified - so I've made it print
    a warning and end the loop if xas_retry() or xa_is_value() return true (which
    takes care of the !page case too). Possibly I could just leave it to fault in
    this case and not check.

    If PageHuge(page) is true, I presume I need to support that too. How do I
    find out how big the page is?

    David

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