linux-kernel - Re: Cache flush issue with page_mapping

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Date:   Thu, 28 May 2020 07:20:25 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Huang Ying <ying.huang@...el.com>,
        linux-kernel@...r.kernel.org,
        Steven Capper <steve.capper@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Rabin Vincent <rabinv@...s.com>,
        linux-arm-kernel@...ts.infradead.org, rmk+kernel@....linux.org.uk,
        Guo Ren <guoren@...nel.org>, linux-mips@...r.kernel.org,
        Ralf Baechle <ralf@...ux-mips.org>,
        Paul Burton <paulburton@...nel.org>,
        James Hogan <jhogan@...nel.org>,
        Ley Foon Tan <lftan@...era.com>,
        nios2-dev@...ts.rocketboards.org, linux-parisc@...r.kernel.org,
        Helge Deller <deller@....de>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>, linux-sh@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        sparclinux@...r.kernel.org, Guan Xuetao <gxt@....edu.cn>,
        linux-xtensa@...ux-xtensa.org, Max Filippov <jcmvbkbc@...il.com>,
        Chris Zankel <chris@...kel.net>
Subject: Re: Cache flush issue with page_mapping_file() and swap back shmem
 page ?

On Wed, May 27, 2020 at 08:46:22PM -0700, Hugh Dickins wrote:
> Hi Jerome,
> 
> On Wed, 27 May 2020, Jerome Glisse wrote:
> > So any arch code which uses page_mapping_file() might get the wrong
> > answer, this function will return NULL for a swap backed page which
> > can be a shmem pages. But shmem pages can still be shared among
> > multiple process (and possibly at different virtual addresses if
> > mremap was use).
> > 
> > Attached is a patch that changes page_mapping_file() to return the
> > shmem mapping for swap backed shmem page. I have not tested it (no
> > way for me to test all those architecture) and i spotted this while
> > working on something else. So i hope someone can take a closer look.
> 
> I'm certainly no expert on flush_dcache_page() and friends, but I'd
> be very surprised if such a problem exists, yet has gone unnoticed
> for so long.  page_mapping_file() itself is fairly new, added when
> a risk of crashing on a race with swapoff came in: but the previous
> use of page_mapping() would have suffered equally if there were such
> a cache flushinhg problem here.
> 
> And I'm afraid your patch won't do anything to help if there is a
> problem: very soon after shmem calls add_to_swap_cache(), it calls
> shmem_delete_from_page_cache(), which sets page->mapping to NULL.
> 
> But I can assure you that a shmem page (unlike an anon page) is never
> put into swap cache while it is mapped into userspace, and never
> mapped into userspace while it is still in swap cache: does that help?
> 

You are right i missed/forgot the part where shmem is never swapcache
and mapped at the same time, thus page_mapping_file() can return NULL
for those as they can no longer have alias mapping.

Thank you Hugh
Jérôme