Re: [PATCH RESEND] mm, thp: bail out early in collapse_file for writeback page

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Fri, Oct 22, 2021 at 10:30:52AM +0800, Rongwei Wang wrote:
> Currently collapse_file does not explicitly check PG_writeback, instead,
> page_has_private and try_to_release_page are used to filter writeback
> pages. This does not work for xfs with blocksize equal to or larger
> than pagesize, because in such case xfs has no page->private.
> 
> This makes collapse_file bail out early for writeback page. Otherwise,
> xfs end_page_writeback will panic as follows.
> 
> page:fffffe00201bcc80 refcount:0 mapcount:0 mapping:ffff0003f88c86a8 index:0x0 pfn:0x84ef32
> aops:xfs_address_space_operations [xfs] ino:30000b7 dentry name:"libtest.so"
> flags: 0x57fffe0000008027(locked|referenced|uptodate|active|writeback)
> raw: 57fffe0000008027 ffff80001b48bc28 ffff80001b48bc28 ffff0003f88c86a8
> raw: 0000000000000000 0000000000000000 00000000ffffffff ffff0000c3e9a000
> page dumped because: VM_BUG_ON_PAGE(((unsigned int) page_ref_count(page) + 127u <= 127u))
> page->mem_cgroup:ffff0000c3e9a000
> ------------[ cut here ]------------
> kernel BUG at include/linux/mm.h:1212!
> Internal error: Oops - BUG: 0 [#1] SMP
> Modules linked in:
> BUG: Bad page state in process khugepaged  pfn:84ef32
>  xfs(E)
> page:fffffe00201bcc80 refcount:0 mapcount:0 mapping:0 index:0x0 pfn:0x84ef32
>  libcrc32c(E) rfkill(E) aes_ce_blk(E) crypto_simd(E) ...
> CPU: 25 PID: 0 Comm: swapper/25 Kdump: loaded Tainted: ...
> pstate: 60400005 (nZCv daif +PAN -UAO -TCO BTYPE=--)
> pc : end_page_writeback+0x1c0/0x214
> lr : end_page_writeback+0x1c0/0x214
> sp : ffff800011ce3cc0
> x29: ffff800011ce3cc0 x28: 0000000000000000
> x27: ffff000c04608040 x26: 0000000000000000
> x25: ffff000c04608040 x24: 0000000000001000
> x23: ffff0003f88c8530 x22: 0000000000001000
> x21: ffff0003f88c8530 x20: 0000000000000000
> x19: fffffe00201bcc80 x18: 0000000000000030
> x17: 0000000000000000 x16: 0000000000000000
> x15: ffff000c018f9760 x14: ffffffffffffffff
> x13: ffff8000119d72b0 x12: ffff8000119d6ee3
> x11: ffff8000117b69b8 x10: 00000000ffff8000
> x9 : ffff800010617534 x8 : 0000000000000000
> x7 : ffff8000114f69b8 x6 : 000000000000000f
> x5 : 0000000000000000 x4 : 0000000000000000
> x3 : 0000000000000400 x2 : 0000000000000000
> x1 : 0000000000000000 x0 : 0000000000000000
> Call trace:
>  end_page_writeback+0x1c0/0x214
>  iomap_finish_page_writeback+0x13c/0x204
>  iomap_finish_ioend+0xe8/0x19c
>  iomap_writepage_end_bio+0x38/0x50
>  bio_endio+0x168/0x1ec
>  blk_update_request+0x278/0x3f0
>  blk_mq_end_request+0x34/0x15c
>  virtblk_request_done+0x38/0x74 [virtio_blk]
>  blk_done_softirq+0xc4/0x110
>  __do_softirq+0x128/0x38c
>  __irq_exit_rcu+0x118/0x150
>  irq_exit+0x1c/0x30
>  __handle_domain_irq+0x8c/0xf0
>  gic_handle_irq+0x84/0x108
>  el1_irq+0xcc/0x180
>  arch_cpu_idle+0x18/0x40
>  default_idle_call+0x4c/0x1a0
>  cpuidle_idle_call+0x168/0x1e0
>  do_idle+0xb4/0x104
>  cpu_startup_entry+0x30/0x9c
>  secondary_start_kernel+0x104/0x180
> Code: d4210000 b0006161 910c8021 94013f4d (d4210000)
> ---[ end trace 4a88c6a074082f8c ]---
> Kernel panic - not syncing: Oops - BUG: Fatal exception in interrupt
> 
> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
> Suggested-by: Yang Shi <shy828301@gmail.com>
> Signed-off-by: Xu Yu <xuyu@linux.alibaba.com>
> Signed-off-by: Rongwei Wang <rongwei.wang@linux.alibaba.com>
> Cc: <stable@vger.kernel.org>
> Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Reviewed-by: Yang Shi <shy828301@gmail.com>

Okay, it makes sense:

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

I would also add a sentance in the commit message that PG_writeback
setting is serialized with page_lock(). I have not had experience with
writeback and it was something that I had to dig up.

> ---
>  mm/khugepaged.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 045cc579f724..48de4e1b0783 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1763,6 +1763,10 @@ static void collapse_file(struct mm_struct *mm,
>  				filemap_flush(mapping);
>  				result = SCAN_FAIL;
>  				goto xa_unlocked;
> +			} else if (PageWriteback(page)) {
> +				xas_unlock_irq(&xas);
> +				result = SCAN_FAIL;
> +				goto xa_unlocked;
>  			} else if (trylock_page(page)) {
>  				get_page(page);
>  				xas_unlock_irq(&xas);
> @@ -1798,7 +1802,8 @@ static void collapse_file(struct mm_struct *mm,
>  			goto out_unlock;
>  		}
>  
> -		if (!is_shmem && PageDirty(page)) {
> +		if (!is_shmem && (PageDirty(page) ||
> +				  PageWriteback(page))) {
>  			/*
>  			 * khugepaged only works on read-only fd, so this
>  			 * page is dirty because it hasn't been flushed
> -- 
> 2.27.0
> 
> 

-- 
 Kirill A. Shutemov