Re: [PATCH v2] mm: khugepaged: fix call hpage_collapse_scan_file() for anonymous vma

[David Hildenbrand <david@redhat.com>]

On 11.01.25 04:45, Liu Shixin wrote:
> syzkaller reported such a BUG_ON():
> 
>   ------------[ cut here ]------------
>   kernel BUG at mm/khugepaged.c:1835!
>   Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
>   ...
>   CPU: 6 UID: 0 PID: 8009 Comm: syz.15.106 Kdump: loaded Tainted: G        W          6.13.0-rc6 #22
>   Tainted: [W]=WARN
>   Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
>   pstate: 00400005 (nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>   pc : collapse_file+0xa44/0x1400
>   lr : collapse_file+0x88/0x1400
>   sp : ffff80008afe3a60
>   ...
>   Call trace:
>    collapse_file+0xa44/0x1400 (P)
>    hpage_collapse_scan_file+0x278/0x400
>    madvise_collapse+0x1bc/0x678
>    madvise_vma_behavior+0x32c/0x448
>    madvise_walk_vmas.constprop.0+0xbc/0x140
>    do_madvise.part.0+0xdc/0x2c8
>    __arm64_sys_madvise+0x68/0x88
>    invoke_syscall+0x50/0x120
>    el0_svc_common.constprop.0+0xc8/0xf0
>    do_el0_svc+0x24/0x38
>    el0_svc+0x34/0x128
>    el0t_64_sync_handler+0xc8/0xd0
>    el0t_64_sync+0x190/0x198
> 
> This indicates that the pgoff is unaligned. After analysis, I confirm
> the vma is mapped to /dev/zero. Such a vma certainly has vm_file, but
> it is set to anonymous by mmap_zero(). So even if it's mmapped by
> 2m-unaligned, it can pass the check in thp_vma_allowable_order() as it
> is an anonymous-mmap, but then be collapsed as a file-mmap.
> 
> It seems the problem has existed for a long time, but actually, since
> we have khugepaged_max_ptes_none check before, we will skip collapse it
> as it is /dev/zero and so has no present page. But commit d8ea7cc8547c
> limit the check for only khugepaged, so the BUG_ON() can be triggered
> by madvise_collapse().
> 
> Add vma_is_anonymous() check to make such vma be processed by
> hpage_collapse_scan_pmd().
> 
> Fixes: d8ea7cc8547c ("mm/khugepaged: add flag to predicate khugepaged-only behavior")
> Signed-off-by: Liu Shixin <liushixin2@huawei.com>
> ---
> v1->v2: Remove the redundant vm_file check, suggested by Matthew Wilcox.
> 
>   mm/khugepaged.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 653dbb1ff05c..bad1e130eda8 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2422,7 +2422,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
>   			VM_BUG_ON(khugepaged_scan.address < hstart ||
>   				  khugepaged_scan.address + HPAGE_PMD_SIZE >
>   				  hend);
> -			if (IS_ENABLED(CONFIG_SHMEM) && vma->vm_file) {
> +			if (IS_ENABLED(CONFIG_SHMEM) && !vma_is_anonymous(vma)) {
>   				struct file *file = get_file(vma->vm_file);
>   				pgoff_t pgoff = linear_page_index(vma,
>   						khugepaged_scan.address);
> @@ -2768,7 +2768,7 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
>   		mmap_assert_locked(mm);
>   		memset(cc->node_load, 0, sizeof(cc->node_load));
>   		nodes_clear(cc->alloc_nmask);
> -		if (IS_ENABLED(CONFIG_SHMEM) && vma->vm_file) {
> +		if (IS_ENABLED(CONFIG_SHMEM) && !vma_is_anonymous(vma)) {
>   			struct file *file = get_file(vma->vm_file);
>   			pgoff_t pgoff = linear_page_index(vma, addr);
>   

Acked-by: David Hildenbrand <david@redhat.com>

The whole CONFIG_SHMEM is all weird and needs to be cleaned up ... at 
some point.

BTW, do we correctly handle MAP_PRIVATE of these files, where we would 
have vma->vm_file and !vma_is_anonymous(vma), but could end up having 
anonymous pages in there?

-- 
Cheers,

David / dhildenb