Re: [PATCH] mm: page_alloc: fix memcg accounting leak in speculative cache lookup

[Matthew Wilcox <willy@infradead.org>]

On Tue, Mar 23, 2021 at 03:02:32PM -0400, Johannes Weiner wrote:
> >From f6f062a3ec46f4fb083dcf6792fde9723f18cfc5 Mon Sep 17 00:00:00 2001
> From: Johannes Weiner <hannes@cmpxchg.org>
> Date: Fri, 19 Mar 2021 02:17:00 -0400
> Subject: [PATCH] mm: page_alloc: fix allocation imbalances from speculative
>  cache lookup
> 
> When the freeing of a higher-order page block (non-compound) races
> with a speculative page cache lookup, __free_pages() needs to leave
> the first order-0 page in the chunk to the lookup but free the buddy
> pages that the lookup doesn't know about separately.
> 
> There are currently two problems with it:
> 
> 1. It checks PageHead() to see whether we're dealing with a compound
>    page after put_page_testzero(). But the speculative lookup could
>    have freed the page after our put and cleared PageHead, in which
>    case we would double free the tail pages.
> 
>    To fix this, test PageHead before the put and cache the result for
>    afterwards.
> 
> 2. If such a higher-order page is charged to a memcg (e.g. !vmap
>    kernel stack)), only the first page of the block has page->memcg
>    set. That means we'll uncharge only one order-0 page from the
>    entire block, and leak the remainder.
> 
>    To fix this, add a split_page_memcg() before it starts freeing tail
>    pages, to ensure they all have page->memcg set up.
> 
> While at it, also update the comments a bit to clarify what exactly is
> happening to the page during that race.
> 
> Fixes: e320d3012d25 mm/page_alloc.c: fix freeing non-compound pages
> Reported-by: Hugh Dickins <hughd@google.com>
> Reported-by: Matthew Wilcox <willy@infradead.org>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Cc: <stable@vger.kernel.org> # 5.10+

This version makes me happy.

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Thanks for fixing my buggy fix.