Re: [PATCH 6.12.y] mm: khugepaged: fix call hpage_collapse_scan_file() for anonymous vma

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Jul 29, 2025 at 09:03:47AM +0000, Jakub Acs wrote:
> From: Liu Shixin <liushixin2@huawei.com>
> 
> commit f1897f2f08b28ae59476d8b73374b08f856973af upstream.
> 
> 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().
> 
> Link: https://lkml.kernel.org/r/20250111034511.2223353-1-liushixin2@huawei.com
> Fixes: d8ea7cc8547c ("mm/khugepaged: add flag to predicate khugepaged-only behavior")
> Signed-off-by: Liu Shixin <liushixin2@huawei.com>
> Reviewed-by: Yang Shi <yang@os.amperecomputing.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Cc: Chengming Zhou <chengming.zhou@linux.dev>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
> Cc: Mattew Wilcox <willy@infradead.org>
> Cc: Muchun Song <muchun.song@linux.dev>
> Cc: Nanyong Sun <sunnanyong@huawei.com>
> Cc: Qi Zheng <zhengqi.arch@bytedance.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> [acsjakub: backport, clean apply]
> Cc: Jakub Acs <acsjakub@amazon.de>

You need to sign off on patches you forward on.  Please fix that up and
resend all of these.

thanks,

greg -h