Re: kernel BUG at mm/page-writeback.c:2241 [ BUG_ON(PageWriteback(page); ]

[Qian Cai]

On Thu, 2020-10-22 at 01:49 +0100, Matthew Wilcox wrote:
> On Wed, Oct 21, 2020 at 08:30:18PM -0400, Qian Cai wrote:
> > Today's linux-next starts to trigger this wondering if anyone has any clue.
> 
> I've seen that occasionally too.  I changed that BUG_ON to VM_BUG_ON_PAGE
> to try to get a clue about it.  Good to know it's not the THP patches
> since they aren't in linux-next.
> 
> I don't understand how it can happen.  We have the page locked, and then we
> do:
> 
>                         if (PageWriteback(page)) {
>                                 if (wbc->sync_mode != WB_SYNC_NONE)
>                                         wait_on_page_writeback(page);
>                                 else
>                                         goto continue_unlock;
>                         }
> 
>                         VM_BUG_ON_PAGE(PageWriteback(page), page);
> 
> Nobody should be able to put this page under writeback while we have it
> locked ... right?  The page can be redirtied by the code that's supposed
> to be writing it back, but I don't see how anyone can make PageWriteback
> true while we're holding the page lock.

It happened again on today's linux-next:

[ 7613.579890][T55770] page:00000000a4b35e02 refcount:3 mapcount:0 mapping:00000000457ceb87 index:0x3e pfn:0x1cef4e
[ 7613.590594][T55770] aops:xfs_address_space_operations ino:805d85a dentry name:"doio.f1.55762"
[ 7613.599192][T55770] flags: 0xbfffc0000000bf(locked|waiters|referenced|uptodate|dirty|lru|active)
[ 7613.608596][T55770] raw: 00bfffc0000000bf ffffea0005027d48 ffff88810eaec030 ffff888231f3a6a8
[ 7613.617101][T55770] raw: 000000000000003e 0000000000000000 00000003ffffffff ffff888143724000
[ 7613.625590][T55770] page dumped because: VM_BUG_ON_PAGE(PageWriteback(page))
[ 7613.632695][T55770] page->mem_cgroup:ffff888143724000