On 10/26/20 7:13 AM, Matthew Wilcox wrote:
> On Mon, Oct 26, 2020 at 10:49:48AM +0100, Jan Kara wrote:
>> On Thu 22-10-20 01:49:06, 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.
>>
>> FWIW here's very similar report for ext4 [1] and I strongly suspect this
>> started happening after Linus' rewrite of the page bit waiting logic. Linus
>> thinks it's preexisting bug which just got exposed by his changes (which is
>> possible). I've been searching a culprit for some time but so far I failed.
>> It's good to know it isn't ext4 specific so we should be searching in the
>> generic code ;). So far I was concentrating more on ext4 bits...
>>
>> Honza
>>
>> [1] https://lore.kernel.org/lkml/000000000000d3a33205add2f7b2@xxxxxxxxxx/
>
> Oh good, I was wondering if it was an XFS bug ;-)
>
> I hope Qian gets it to reproduce soon with the assert because that will
> tell us whether it's a spurious wakeup or someone calling SetPageWriteback
> without holding the page lock.
I've tried to reproduce this as well, to no avail. Qian, could you perhaps
detail the setup? What kind of storage, kernel config, compiler, etc.
--
Jens Axboe
