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SubjectRe: [PATCH v2] page_alloc: Fix freeing non-compound pages
On Tue, Sep 29, 2020 at 10:26:22AM +0300, Mike Rapoport wrote:
> This sentence presumes existing description/prior knowledge about
> put_page().
>
> Maybe
>
> This function can free multi-page allocations that were not allocated
> with %__GFP_COMP, unlike put_page() that would free only the first page
> in such case. __free_pages() does not ...

Thanks. After waking up this morning I did a more extensive rewrite:

/**
* __free_pages - Free pages allocated with alloc_pages().
* @page: The page pointer returned from alloc_pages().
* @order: The order of the allocation.
*
* This function can free multi-page allocations that are not compound
* pages. It does not check that the @order passed in matches that of
* the allocation, so it is easy to leak memory. Freeing more memory
* than was allocated will probably emit a warning.
*
* If the last reference to this page is speculative, it will be released
* by put_page() which only frees the first page of a non-compound
* allocation. To prevent the remaining pages from being leaked, we free
* the subsequent pages here. If you want to use the page's reference
* count to decide when to free the allocation, you should allocate a
* compound page, and use put_page() instead of __free_pages().
*
* Context: May be called in interrupt context or holding a normal
* spinlock, but not in NMI context or while holding a raw spinlock.
*/

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