Re: [PATCH v2] page_alloc: Fix freeing non-compound pages

[Matthew Wilcox]

On Mon, Sep 28, 2020 at 06:03:07PM -0700, Andrew Morton wrote:
> Well that's weird and scary looking.  `page' has non-zero refcount yet
> we go and free random followon pages.  Methinks it merits an
> explanatory comment?

Here's some kernel-doc.  Opinions?

/**
 * __free_pages - Free pages allocated with alloc_pages().
 * @page: The page pointer returned from alloc_pages().
 * @order: The order of the allocation.
 *
 * This function differs from put_page() in that it can free multi-page
 * allocations that were not allocated with %__GFP_COMP.  This function
 * does not check that the @order passed in matches that of the
 * allocation, so it is possible to leak memory.  Freeing more memory than
 * was allocated will probably be warned about by other debugging checks.
 *
 * It is only safe to use the page reference count to determine when
 * to free an allocation if you use %__GFP_COMP (in which case, you may
 * as well use put_page() to free the page).  Another thread may have a
 * speculative reference to the first page, but it has no way of knowing
 * about the rest of the allocation, so we have to free all but the
 * first page here.
 *
 * Context: May be called in interrupt context but not NMI context.
 */