linux-kernel - Re: [PATCH -V2 -mm] mm: Fix races between swapoff and flush dcache

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Date:   Tue, 06 Mar 2018 08:47:37 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Minchan Kim <minchan@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Dave Hansen <dave.hansen@...el.com>,
        "Chen Liqin" <liqin.linux@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        "James E.J. Bottomley" <jejb@...isc-linux.org>,
        Guan Xuetao <gxt@...c.pku.edu.cn>,
        "David S. Miller" <davem@...emloft.net>,
        Chris Zankel <chris@...kel.net>,
        Vineet Gupta <vgupta@...opsys.com>,
        Ley Foon Tan <lftan@...era.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH -V2 -mm] mm: Fix races between swapoff and flush dcache

"Huang, Ying" <ying.huang@...el.com> writes:

> From: Huang Ying <ying.huang@...el.com>
>
> From commit 4b3ef9daa4fc ("mm/swap: split swap cache into 64MB
> trunks") on, after swapoff, the address_space associated with the swap
> device will be freed.  So page_mapping() users which may touch the
> address_space need some kind of mechanism to prevent the address_space
> from being freed during accessing.
>
> The dcache flushing functions (flush_dcache_page(), etc) in
> architecture specific code may access the address_space of swap device
> for anonymous pages in swap cache via page_mapping() function.  But in
> some cases there are no mechanisms to prevent the swap device from
> being swapoff, for example,
>
> CPU1					CPU2
> __get_user_pages()			swapoff()
>   flush_dcache_page()
>     mapping = page_mapping()
>       ...				  exit_swap_address_space()
>       ...				    kvfree(spaces)
>       mapping_mapped(mapping)
>
> The address space may be accessed after being freed.
>
> But from cachetlb.txt and Russell King, flush_dcache_page() only care
> about file cache pages, for anonymous pages, flush_anon_page() should
> be used.  The implementation of flush_dcache_page() in all
> architectures follows this too.  They will check whether
> page_mapping() is NULL and whether mapping_mapped() is true to
> determine whether to flush the dcache immediately.  And they will use
> interval tree (mapping->i_mmap) to find all user space mappings.
> While mapping_mapped() and mapping->i_mmap isn't used by anonymous
> pages in swap cache at all.
>
> So, to fix the race between swapoff and flush dcache, __page_mapping()
> is add to return the address_space for file cache pages and NULL
> otherwise.  All page_mapping() invoking in flush dcache functions are
> replaced with __page_mapping().

Sorry, I just found I forgot replacing __page_mapping() to
page_mapping_file() in the above paragraph.  Could you help me to change
it in place?  Or I should resend the patch with the updated description?

Best Regards,
Huang, Ying

[snip]