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Date: Wed, 7 Dec 2022 14:40:34 -0800
From: Yang Shi <shy828301@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
Andrew Morton <akpm@...ux-foundation.org>,
Peter Xu <peterx@...hat.com>, Hugh Dickins <hughd@...gle.com>,
Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH v2] mm/swap: fix SWP_PFN_BITS with CONFIG_PHYS_ADDR_T_64BIT
on 32bit
On Tue, Dec 6, 2022 at 2:57 AM David Hildenbrand <david@...hat.com> wrote:
>
> We use "unsigned long" to store a PFN in the kernel and phys_addr_t to
> store a physical address.
>
> On a 64bit system, both are 64bit wide. However, on a 32bit system, the
> latter might be 64bit wide. This is, for example, the case on x86 with
> PAE: phys_addr_t and PTEs are 64bit wide, while "unsigned long" only
> spans 32bit.
>
> The current definition of SWP_PFN_BITS without MAX_PHYSMEM_BITS misses
> that case, and assumes that the maximum PFN is limited by an 32bit
> phys_addr_t. This implies, that SWP_PFN_BITS will currently only be able to
> cover 4 GiB - 1 on any 32bit system with 4k page size, which is wrong.
Thanks for debugging this. IIUC this means even swap is actually
broken on x86_32 + PAE?
>
> Let's rely on the number of bits in phys_addr_t instead, but make sure to
> not exceed the maximum swap offset, to not make the BUILD_BUG_ON() in
> is_pfn_swap_entry() unhappy. Note that swp_entry_t is effectively an
> unsigned long and the maximum swap offset shares that value with the
> swap type.
>
> For example, on an 8 GiB x86 PAE system with a kernel config based on
> Debian 11.5 (-> CONFIG_FLATMEM=y, CONFIG_X86_PAE=y), we will currently fail
> removing migration entries (remove_migration_ptes()), because
> mm/page_vma_mapped.c:check_pte() will fail to identify a PFN match as
> swp_offset_pfn() wrongly masks off PFN bits. For example,
> split_huge_page_to_list()->...->remap_page() will leave migration
> entries in place and continue to unlock the page.
>
> Later, when we stumble over these migration entries (e.g., via
> /proc/self/pagemap), pfn_swap_entry_to_page() will BUG_ON() because
> these migration entries shouldn't exist anymore and the page was
> unlocked.
>
> [ 33.067591] kernel BUG at include/linux/swapops.h:497!
> [ 33.067597] invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
> [ 33.067602] CPU: 3 PID: 742 Comm: cow Tainted: G E 6.1.0-rc8+ #16
> [ 33.067605] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.0-1.fc36 04/01/2014
> [ 33.067606] EIP: pagemap_pmd_range+0x644/0x650
> [ 33.067612] Code: 00 00 00 00 66 90 89 ce b9 00 f0 ff ff e9 ff fb ff ff 89 d8 31 db e8 48 c6 52 00 e9 23 fb ff ff e8 61 83 56 00 e9 b6 fe ff ff <0f> 0b bf 00 f0 ff ff e9 38 fa ff ff 3e 8d 74 26 00 55 89 e5 57 31
> [ 33.067615] EAX: ee394000 EBX: 00000002 ECX: ee394000 EDX: 00000000
> [ 33.067617] ESI: c1b0ded4 EDI: 00024a00 EBP: c1b0ddb4 ESP: c1b0dd68
> [ 33.067619] DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068 EFLAGS: 00010246
> [ 33.067624] CR0: 80050033 CR2: b7a00000 CR3: 01bbbd20 CR4: 00350ef0
> [ 33.067625] Call Trace:
> [ 33.067628] ? madvise_free_pte_range+0x720/0x720
> [ 33.067632] ? smaps_pte_range+0x4b0/0x4b0
> [ 33.067634] walk_pgd_range+0x325/0x720
> [ 33.067637] ? mt_find+0x1d6/0x3a0
> [ 33.067641] ? mt_find+0x1d6/0x3a0
> [ 33.067643] __walk_page_range+0x164/0x170
> [ 33.067646] walk_page_range+0xf9/0x170
> [ 33.067648] ? __kmem_cache_alloc_node+0x2a8/0x340
> [ 33.067653] pagemap_read+0x124/0x280
> [ 33.067658] ? default_llseek+0x101/0x160
> [ 33.067662] ? smaps_account+0x1d0/0x1d0
> [ 33.067664] vfs_read+0x90/0x290
> [ 33.067667] ? do_madvise.part.0+0x24b/0x390
> [ 33.067669] ? debug_smp_processor_id+0x12/0x20
> [ 33.067673] ksys_pread64+0x58/0x90
> [ 33.067675] __ia32_sys_ia32_pread64+0x1b/0x20
> [ 33.067680] __do_fast_syscall_32+0x4c/0xc0
> [ 33.067683] do_fast_syscall_32+0x29/0x60
> [ 33.067686] do_SYSENTER_32+0x15/0x20
> [ 33.067689] entry_SYSENTER_32+0x98/0xf1
>
> Decrease the indentation level of SWP_PFN_BITS and SWP_PFN_MASK to keep
> it readable and consistent.
>
> Fixes: 0d206b5d2e0d ("mm/swap: add swp_offset_pfn() to fetch PFN from swap entry")
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Peter Xu <peterx@...hat.com>
> Cc: Yang Shi <shy828301@...il.com>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>
> v1 -> v2:
> * Rely on sizeof(phys_addr_t) and min_t() instead.
> * Survives my various cross compilations and testing on x86 PAE.
Reviewed-by: Yang Shi <shy828301@...il.com>
>
> ---
> include/linux/swapops.h | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index 86b95ccb81bb..424d586ed1ca 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -33,11 +33,13 @@
> * can use the extra bits to store other information besides PFN.
> */
> #ifdef MAX_PHYSMEM_BITS
> -#define SWP_PFN_BITS (MAX_PHYSMEM_BITS - PAGE_SHIFT)
> +#define SWP_PFN_BITS (MAX_PHYSMEM_BITS - PAGE_SHIFT)
> #else /* MAX_PHYSMEM_BITS */
> -#define SWP_PFN_BITS (BITS_PER_LONG - PAGE_SHIFT)
> +#define SWP_PFN_BITS min_t(phys_addr_t, \
> + sizeof(phys_addr_t) * 8 - PAGE_SHIFT, \
> + SWP_TYPE_SHIFT)
> #endif /* MAX_PHYSMEM_BITS */
> -#define SWP_PFN_MASK (BIT(SWP_PFN_BITS) - 1)
> +#define SWP_PFN_MASK (BIT(SWP_PFN_BITS) - 1)
>
> /**
> * Migration swap entry specific bitfield definitions. Layout:
>
> base-commit: 76dcd734eca23168cb008912c0f69ff408905235
> --
> 2.38.1
>
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