Re: [PATCH] arm64: fix MAX_ORDER for 64K pagesize

From: Mark Salter
Date: Thu Jun 19 2014 - 14:12:47 EST


On Tue, 2014-06-17 at 20:32 +0200, Michal Nazarewicz wrote:
> On Wed, Jun 11 2014, David Rientjes wrote:
> > On Wed, 11 Jun 2014, Mark Salter wrote:
> >
> >> With a kernel configured with ARM64_64K_PAGES && !TRANSPARENT_HUGEPAGE
> >> I get this at early boot:
> >>
> >> SMP: Total of 8 processors activated.
> >> devtmpfs: initialized
> >> Unable to handle kernel NULL pointer dereference at virtual address 00000008
> >> pgd = fffffe0000050000
> >> [00000008] *pgd=00000043fba00003, *pmd=00000043fba00003, *pte=00e0000078010407
> >> Internal error: Oops: 96000006 [#1] SMP
> >> Modules linked in:
> >> CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.15.0-rc864k+ #44
> >> task: fffffe03bc040000 ti: fffffe03bc080000 task.ti: fffffe03bc080000
> >> PC is at __list_add+0x10/0xd4
> >> LR is at free_one_page+0x270/0x638
> >> ...
> >> Call trace:
> >> [<fffffe00003ee970>] __list_add+0x10/0xd4
> >> [<fffffe000019c478>] free_one_page+0x26c/0x638
> >> [<fffffe000019c8c8>] __free_pages_ok.part.52+0x84/0xbc
> >> [<fffffe000019d5e8>] __free_pages+0x74/0xbc
> >> [<fffffe0000c01350>] init_cma_reserved_pageblock+0xe8/0x104
> >> [<fffffe0000c24de0>] cma_init_reserved_areas+0x190/0x1e4
> >> [<fffffe0000090418>] do_one_initcall+0xc4/0x154
> >> [<fffffe0000bf0a50>] kernel_init_freeable+0x204/0x2a8
> >> [<fffffe00007520a0>] kernel_init+0xc/0xd4
> >>
> >> This happens in this configuration because __free_one_page() is called
> >> with an order greater than MAX_ORDER, accesses past zone->free_list[]
> >> and passes a bogus list_head to list_add().
> >>
> >> arch/arm64/Kconfig has:
> >>
> >> config FORCE_MAX_ZONEORDER
> >> int
> >> default "14" if (ARM64_64K_PAGES && TRANSPARENT_HUGEPAGE)
> >> default "11"
> >>
> >> So with THP turned off MAX_ORDER == 11 but init_cma_reserved_pageblock()
> >> passes __free_pages() an order of pageblock_order which is based on
> >> (HPAGE_SHIFT - PAGE_SHIFT) which is 13 for 64K pages. I worked around
> >> this by removing the THP test so FORCE_MAX_ZONEORDER is always 14 for
> >> ARM64_64K_PAGES.
> >>
> >> Signed-off-by: Mark Salter <msalter@xxxxxxxxxx>
> >> ---
> >> arch/arm64/Kconfig | 2 +-
> >> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> >> index 7295419..42a334e 100644
> >> --- a/arch/arm64/Kconfig
> >> +++ b/arch/arm64/Kconfig
> >> @@ -269,7 +269,7 @@ config XEN
> >>
> >> config FORCE_MAX_ZONEORDER
> >> int
> >> - default "14" if (ARM64_64K_PAGES && TRANSPARENT_HUGEPAGE)
> >> + default "14" if ARM64_64K_PAGES
> >> default "11"
> >>
> >> endmenu
> >
> > Any reason to not switch this to
> >
> > ARM64_64K_PAGES && TRANSPARENT_HUGEPAGE && CMA
> >
> > instead? If pageblock_order > MAX_ORDER because of
> > HPAGE_SHIFT > PAGE_SHIFT, then cma is always going to be passing a
> > too-large-order to free_pages_prepare() via this path.
> >
> > Adding Michal and Marek to the cc.
>
> The correct fix would be to change init_cma_reserved_pageblock such that
> it checks whether pageblock_order > MAX_ORDER and if so frees each max
> order page of the pageblock individually:
>
> --------- >8 ---------------------------------------------------------
> From: Michal Nazarewicz <mina86@xxxxxxxxxx>
> Subject: [PATCH] mm: cma: fix cases where pageblock is bigger then MAX_ORDER
>
> With a kernel configured with ARM64_64K_PAGES && !TRANSPARENT_HUGEPAGE,
> the following is triggered at early boot:
>
> SMP: Total of 8 processors activated.
> devtmpfs: initialized
> Unable to handle kernel NULL pointer dereference at virtual address 00000008
> pgd = fffffe0000050000
> [00000008] *pgd=00000043fba00003, *pmd=00000043fba00003, *pte=00e0000078010407
> Internal error: Oops: 96000006 [#1] SMP
> Modules linked in:
> CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.15.0-rc864k+ #44
> task: fffffe03bc040000 ti: fffffe03bc080000 task.ti: fffffe03bc080000
> PC is at __list_add+0x10/0xd4
> LR is at free_one_page+0x270/0x638
> ...
> Call trace:
> [<fffffe00003ee970>] __list_add+0x10/0xd4
> [<fffffe000019c478>] free_one_page+0x26c/0x638
> [<fffffe000019c8c8>] __free_pages_ok.part.52+0x84/0xbc
> [<fffffe000019d5e8>] __free_pages+0x74/0xbc
> [<fffffe0000c01350>] init_cma_reserved_pageblock+0xe8/0x104
> [<fffffe0000c24de0>] cma_init_reserved_areas+0x190/0x1e4
> [<fffffe0000090418>] do_one_initcall+0xc4/0x154
> [<fffffe0000bf0a50>] kernel_init_freeable+0x204/0x2a8
> [<fffffe00007520a0>] kernel_init+0xc/0xd4
>
> This happens in this configuration because __free_one_page() is called
> with an order greater than MAX_ORDER, accesses past zone->free_list[]
> and passes a bogus list_head to list_add().
>
> arch/arm64/Kconfig has:
>
> config FORCE_MAX_ZONEORDER
> int
> default "14" if (ARM64_64K_PAGES && TRANSPARENT_HUGEPAGE)
> default "11"
>
> So with THP turned off MAX_ORDER == 11 but init_cma_reserved_pageblock()
> passes __free_pages() an order of pageblock_order which is based on
> (HPAGE_SHIFT - PAGE_SHIFT) which is 13 for 64K pages.
>
> Fix the problem by changing init_cma_reserved_pageblock() such that it
> splits pageblock into individual MAX_ORDER pages if pageblock is
> bigger than a MAX_ORDER page.
>
> Signed-off-by: Michal Nazarewicz <mina86@xxxxxxxxxx>
> Reported-by: Mark Salter <msalter@xxxxxxxxxx>
> ---
> mm/page_alloc.c | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5dba293..6e657ce 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -801,7 +801,15 @@ void __init init_cma_reserved_pageblock(struct page *page)
>
> set_page_refcounted(page);
> set_pageblock_migratetype(page, MIGRATE_CMA);
> - __free_pages(page, pageblock_order);
> + if (pageblock_order > MAX_ORDER) {
> + struct page *subpage = p;
> + unsigned count = 1 << (pageblock_order - MAX_ORDER);
> + do {
> + __free_pages(subpage, pageblock_order);
^^^^^^^
MAX_ORDER

> + } while (subpage += MAX_ORDER_NR_PAGES, --count);
> + } else {
> + __free_pages(page, pageblock_order);
> + }
> adjust_managed_page_count(page, pageblock_nr_pages);
> }
> #endif
> --------- >8 ---------------------------------------------------------
>
> Thoughts? This has not been tested and I think it may cause performance
> degradation in some cases since pageblock_order is not always
> a constant, so the comparison may end up not being stripped away even on
> systems where it's always false.
>

This works with the above tweak. So it fixes the problm here, but I was
not sure if we'd get bitten elsewhere by pageblock_order > MAX_ORDER.
It will be slower, but does it only gets called a few time at most at
boot time, right?


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