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    SubjectRe: [PATCH v33 1/4] mm: add a function to get free page blocks
    On Fri, Jun 15, 2018 at 12:43:10PM +0800, Wei Wang wrote:
    > +/**
    > + * get_from_free_page_list - get free page blocks from a free page list
    > + * @order: the order of the free page list to check
    > + * @buf: the array to store the physical addresses of the free page blocks
    > + * @size: the array size
    > + *
    > + * This function offers hints about free pages. There is no guarantee that
    > + * the obtained free pages are still on the free page list after the function
    > + * returns. pfn_to_page on the obtained free pages is strongly discouraged
    > + * and if there is an absolute need for that, make sure to contact MM people
    > + * to discuss potential problems.
    > + *
    > + * The addresses are currently stored to the array in little endian. This
    > + * avoids the overhead of converting endianness by the caller who needs data
    > + * in the little endian format. Big endian support can be added on demand in
    > + * the future.
    > + *
    > + * Return the number of free page blocks obtained from the free page list.
    > + * The maximum number of free page blocks that can be obtained is limited to
    > + * the caller's array size.
    > + */

    Please use:

    * Return: The number of free page blocks obtained from the free page list.

    Also, please include a

    * Context: Any context.

    or

    * Context: Process context.

    or whatever other conetext this function can be called from. Since you're
    taking the lock irqsafe, I assume this can be called from any context, but
    I wonder if it makes sense to have this function callable from interrupt
    context. Maybe this should be callable from process context only.

    > +uint32_t get_from_free_page_list(int order, __le64 buf[], uint32_t size)
    > +{
    > + struct zone *zone;
    > + enum migratetype mt;
    > + struct page *page;
    > + struct list_head *list;
    > + unsigned long addr, flags;
    > + uint32_t index = 0;
    > +
    > + for_each_populated_zone(zone) {
    > + spin_lock_irqsave(&zone->lock, flags);
    > + for (mt = 0; mt < MIGRATE_TYPES; mt++) {
    > + list = &zone->free_area[order].free_list[mt];
    > + list_for_each_entry(page, list, lru) {
    > + addr = page_to_pfn(page) << PAGE_SHIFT;
    > + if (likely(index < size)) {
    > + buf[index++] = cpu_to_le64(addr);
    > + } else {
    > + spin_unlock_irqrestore(&zone->lock,
    > + flags);
    > + return index;
    > + }
    > + }
    > + }
    > + spin_unlock_irqrestore(&zone->lock, flags);
    > + }
    > +
    > + return index;
    > +}

    I wonder if (to address Michael's concern), you shouldn't instead use
    the first free chunk of pages to return the addresses of all the pages.
    ie something like this:

    __le64 *ret = NULL;
    unsigned int max = (PAGE_SIZE << order) / sizeof(__le64);

    for_each_populated_zone(zone) {
    spin_lock_irq(&zone->lock);
    for (mt = 0; mt < MIGRATE_TYPES; mt++) {
    list = &zone->free_area[order].free_list[mt];
    list_for_each_entry_safe(page, list, lru, ...) {
    if (index == size)
    break;
    addr = page_to_pfn(page) << PAGE_SHIFT;
    if (!ret) {
    list_del(...);
    ret = addr;
    }
    ret[index++] = cpu_to_le64(addr);
    }
    }
    spin_unlock_irq(&zone->lock);
    }

    return ret;
    }

    You'll need to return the page to the freelist afterwards, but free_pages()
    should take care of that.

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