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

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Date:   Tue, 29 Sep 2020 04:40:26 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Nick Piggin <npiggin@...il.com>, Hugh Dickins <hughd@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, hch@....de, rppt@...ux.ibm.com,
        rdunlap@...radead.org
Subject: Re: [PATCH v2] page_alloc: Fix freeing non-compound pages

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.
 */