Re: [PATCH v3 v3 2/2] mm, thp: bail out early in collapse_file for writeback page

[Rongwei Wang]

On 10/6/21 10:41 AM, Matthew Wilcox wrote:
> On Wed, Oct 06, 2021 at 10:18:37AM +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.
>
> Could you cut the timestamps out?  They don't add any value.
OK, no problem!
> > [ 6411.448211] page:fffffe00201bcc80 refcount:0 mapcount:0 mapping:ffff0003f88c86a8 index:0x0 pfn:0x84ef32
> > [ 6411.448304] aops:xfs_address_space_operations [xfs] ino:30000b7 dentry name:"libtest.so"
> > [ 6411.448312] flags: 0x57fffe0000008027(locked|referenced|uptodate|active|writeback)
> > [ 6411.448317] raw: 57fffe0000008027 ffff80001b48bc28 ffff80001b48bc28 ffff0003f88c86a8
> > [ 6411.448321] raw: 0000000000000000 0000000000000000 00000000ffffffff ffff0000c3e9a000
> > [ 6411.448324] page dumped because: VM_BUG_ON_PAGE(((unsigned int) page_ref_count(page) + 127u <= 127u))
> > [ 6411.448327] page->mem_cgroup:ffff0000c3e9a000
> > [ 6411.448340] ------------[ cut here ]------------
> > [ 6411.448343] kernel BUG at include/linux/mm.h:1212!
> > [ 6411.449288] Internal error: Oops - BUG: 0 [#1] SMP
> > [ 6411.449786] Modules linked in:
> > [ 6411.449790] BUG: Bad page state in process khugepaged  pfn:84ef32
> > [ 6411.450143]  xfs(E)
> > [ 6411.450459] page:fffffe00201bcc80 refcount:0 mapcount:0 mapping:0 index:0x0 pfn:0x84ef32
> > [ 6411.451361]  libcrc32c(E) rfkill(E) aes_ce_blk(E) crypto_simd(E) ...
> > [ 6411.451387] CPU: 25 PID: 0 Comm: swapper/25 Kdump: loaded Tainted: ...
> > [ 6411.451389] pstate: 60400005 (nZCv daif +PAN -UAO -TCO BTYPE=--)
> > [ 6411.451393] pc : end_page_writeback+0x1c0/0x214
> > [ 6411.451394] lr : end_page_writeback+0x1c0/0x214
> > [ 6411.451395] sp : ffff800011ce3cc0
> > [ 6411.451396] x29: ffff800011ce3cc0 x28: 0000000000000000
> > [ 6411.451398] x27: ffff000c04608040 x26: 0000000000000000
> > [ 6411.451399] x25: ffff000c04608040 x24: 0000000000001000
> > [ 6411.451401] x23: ffff0003f88c8530 x22: 0000000000001000
> > [ 6411.451403] x21: ffff0003f88c8530 x20: 0000000000000000
> > [ 6411.451404] x19: fffffe00201bcc80 x18: 0000000000000030
> > [ 6411.451406] x17: 0000000000000000 x16: 0000000000000000
> > [ 6411.451407] x15: ffff000c018f9760 x14: ffffffffffffffff
> > [ 6411.451409] x13: ffff8000119d72b0 x12: ffff8000119d6ee3
> > [ 6411.451410] x11: ffff8000117b69b8 x10: 00000000ffff8000
> > [ 6411.451412] x9 : ffff800010617534 x8 : 0000000000000000
> > [ 6411.451413] x7 : ffff8000114f69b8 x6 : 000000000000000f
> > [ 6411.451415] x5 : 0000000000000000 x4 : 0000000000000000
> > [ 6411.451416] x3 : 0000000000000400 x2 : 0000000000000000
> > [ 6411.451418] x1 : 0000000000000000 x0 : 0000000000000000
> > [ 6411.451420] Call trace:
> > [ 6411.451421]  end_page_writeback+0x1c0/0x214
> > [ 6411.451424]  iomap_finish_page_writeback+0x13c/0x204
> > [ 6411.451425]  iomap_finish_ioend+0xe8/0x19c
> > [ 6411.451426]  iomap_writepage_end_bio+0x38/0x50
> > [ 6411.451427]  bio_endio+0x168/0x1ec
> > [ 6411.451430]  blk_update_request+0x278/0x3f0
> > [ 6411.451432]  blk_mq_end_request+0x34/0x15c
> > [ 6411.451435]  virtblk_request_done+0x38/0x74 [virtio_blk]
> > [ 6411.451437]  blk_done_softirq+0xc4/0x110
> > [ 6411.451439]  __do_softirq+0x128/0x38c
> > [ 6411.451441]  __irq_exit_rcu+0x118/0x150
> > [ 6411.451442]  irq_exit+0x1c/0x30
> > [ 6411.451445]  __handle_domain_irq+0x8c/0xf0
> > [ 6411.451446]  gic_handle_irq+0x84/0x108
> > [ 6411.451447]  el1_irq+0xcc/0x180
> > [ 6411.451448]  arch_cpu_idle+0x18/0x40
> > [ 6411.451450]  default_idle_call+0x4c/0x1a0
> > [ 6411.451453]  cpuidle_idle_call+0x168/0x1e0
> > [ 6411.451454]  do_idle+0xb4/0x104
> > [ 6411.451455]  cpu_startup_entry+0x30/0x9c
> > [ 6411.451458]  secondary_start_kernel+0x104/0x180
> > [ 6411.451460] Code: d4210000 b0006161 910c8021 94013f4d (d4210000)
> > [ 6411.451462] ---[ end trace 4a88c6a074082f8c ]---
> > [ 6411.451464] Kernel panic - not syncing: Oops - BUG: Fatal exception in interrupt
> >
> > Fixes: eb6ecbed0aa2 ("mm, thp: relax the VM_DENYWRITE constraint on file-backed THPs")
> > Suggested-by: Yang Shi <shy828301@xxxxxxxxx>
> > Signed-off-by: Xu Yu <xuyu@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Rongwei Wang <rongwei.wang@xxxxxxxxxxxxxxxxx>
>
> Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
>
> Although I think the Fixes: line is wrong.  This bug goes all the
> way back to 99cb0dbd47a1.
In fact, I understand it this way: the 99cb0dbd47a1 only introduce 
file-backed THP for VMA with MAP_DENYWRITE, denying write directly not 
to truncate pagecache when it is opened. And eb6ecbed0aa2 will to 
truncate pagecache when DSO is opened by a writer, then will trigger ...

It seems 'Fixes: eb6ecbed0aa2' is more reasonable.
>