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

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Date:   Wed, 30 Sep 2020 12:17:55 +0300
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        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, rdunlap@...radead.org
Subject: Re: [PATCH v2] page_alloc: Fix freeing non-compound pages

On Tue, Sep 29, 2020 at 03:06:22PM +0100, Matthew Wilcox wrote:
> 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:

I like this one

Acked-by: Mike Rapoport <rppt@...ux.ibm.com>

> /**
>  * __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.
>  */
> 

-- 
Sincerely yours,
Mike.