Re: kernel BUG at fs/ext4/inode.c:LINE!

[Linus Torvalds]

On Tue, Nov 24, 2020 at 12:16 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> So my s/if/while/ suggestion is wrong and we need to do something to
> prevent spurious wakeups.  Unless we bury the spurious wakeup logic
> inside wait_on_page_writeback() ...

We can certainly make the "if()" in that loop be a "while()'.

That's basically what the old code did - simply by virtue of the
wakeup not happening if the writeback bit was set in
wake_page_function():

        if (test_bit(key->bit_nr, &key->page->flags))
                return -1;

of course, the race was still there - because the writeback bit might
be clear at that point, but another CPU would reallocate and dirty it,
and then autoremove_wake_function() would happen anyway.

But back in the bad old days, the wait_on_page_bit_common() code would
then double-check in a loop, so it would catch that case, re-insert
itself on the wait queue, and try again. Except for the DROP case,
which isn't used by writeback.

Anyway, making that "if()" be a "while()" in wait_on_page_writeback()
would basically re-introduce that old behavior. I don't really care,
because it was the lock bit that really mattered, the writeback bit is
not really all that interesting (except from a "let's fix this bug"
angle)

I'm not 100% sure I like the fragility of this writeback thing.

Anyway, I'm certainly happy with either model, whether it be an added
while() in wait_on_page_writeback(), or it be the page reference count
in end_page_writeback().

Strong opinions?

            Linus