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Message-ID: <20190104215636.GM19981@redhat.com>
Date: Fri, 4 Jan 2019 16:56:36 -0500
From: Andrea Arcangeli <aarcange@...hat.com>
To: Jan Stancek <jstancek@...hat.com>
Cc: linux-mm@...ck.org, lersek@...hat.com, alex.williamson@...hat.com,
rientjes@...gle.com, kirill@...temov.name,
mgorman@...hsingularity.net, mhocko@...e.com,
linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2] mm: page_mapped: don't assume compound page is huge
or THP
[ CC'ed Andrew for potential inclusion in -mm ]
On Fri, Nov 30, 2018 at 01:06:57PM +0100, Jan Stancek wrote:
> LTP proc01 testcase has been observed to rarely trigger crashes
> on arm64:
> page_mapped+0x78/0xb4
> stable_page_flags+0x27c/0x338
> kpageflags_read+0xfc/0x164
> proc_reg_read+0x7c/0xb8
> __vfs_read+0x58/0x178
> vfs_read+0x90/0x14c
> SyS_read+0x60/0xc0
>
> Issue is that page_mapped() assumes that if compound page is not
> huge, then it must be THP. But if this is 'normal' compound page
> (COMPOUND_PAGE_DTOR), then following loop can keep running
> (for HPAGE_PMD_NR iterations) until it tries to read from memory
> that isn't mapped and triggers a panic:
> for (i = 0; i < hpage_nr_pages(page); i++) {
> if (atomic_read(&page[i]._mapcount) >= 0)
> return true;
> }
>
> I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only
> with a custom kernel module [1] which:
> - allocates compound page (PAGEC) of order 1
> - allocates 2 normal pages (COPY), which are initialized to 0xff
> (to satisfy _mapcount >= 0)
> - 2 PAGEC page structs are copied to address of first COPY page
> - second page of COPY is marked as not present
> - call to page_mapped(COPY) now triggers fault on access to 2nd
> COPY page at offset 0x30 (_mapcount)
>
> [1] https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash/repro.c
>
> Fix the loop to iterate for "1 << compound_order" pages.
>
> Debugged-by: Laszlo Ersek <lersek@...hat.com>
> Suggested-by: "Kirill A. Shutemov" <kirill@...temov.name>
> Signed-off-by: Jan Stancek <jstancek@...hat.com>
> ---
> mm/util.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> Changes in v2:
> - change the loop instead so we check also mapcount of subpages
Reviewed-by: Andrea Arcangeli <aarcange@...hat.com>
Thanks,
Andrea
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