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

[Yang Shi <yang@os.amperecomputing.com>]

On 1/8/25 11:00 PM, 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>
> ---
>   mm/khugepaged.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 653dbb1ff05c..eb9d240e42e8 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2422,7 +2422,8 @@ 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->vm_file &&
> +			    !vma_is_anonymous(vma)) {

Thanks for catching this. It sounds a little bit weird to have vm_file 
for an anonymous VMA. I'm not sure why we should keep such special case. 
It seems shared mapping is treated as shmem file mapping. So can we set 
vm_file to NULL when mmap'ing /dev/zero for private mapping? Something like:

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 169eed162a7f..fc332efc5c11 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -527,6 +527,7 @@ static int mmap_zero(struct file *file, struct 
vm_area_struct *vma)
         if (vma->vm_flags & VM_SHARED)
                 return shmem_zero_setup(vma);
         vma_set_anonymous(vma);
+       vma->vm_file = NULL;
         return 0;
  }


>   				struct file *file = get_file(vma->vm_file);
>   				pgoff_t pgoff = linear_page_index(vma,
>   						khugepaged_scan.address);
> @@ -2768,7 +2769,8 @@ 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->vm_file &&
> +		    !vma_is_anonymous(vma)) {
>   			struct file *file = get_file(vma->vm_file);
>   			pgoff_t pgoff = linear_page_index(vma, addr);
>