Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page cache

[Song Liu]

On Wed, Sep 29, 2021 at 10:56 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
[...]
> > Now, I am able to crash the system on
> >     find_lock_entries () {
> >      ...
> >        VM_BUG_ON_PAGE(page->index != xas.xa_index, page);
> >     }
> > I guess it is related. I will test more.
>
> That's a bogus VM_BUG_ON.  I have a patch in my tree to delete it.
> Andrew has it too, but for some reason, he hasn't sent it on to Linus.
>
> +++ b/mm/filemap.c
> @@ -2093,7 +2093,6 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,
>                 if (!xa_is_value(page)) {
>                         if (page->index < start)
>                                 goto put;
> -                       VM_BUG_ON_PAGE(page->index != xas.xa_index, page);
>                         if (page->index + thp_nr_pages(page) - 1 > end)
>                                 goto put;
>                         if (!trylock_page(page))

Yes, after removing this line, I am able to see the same bug.

Here is my finding so far:

The issue is NOT caused by concurrent khugepaged:collapse_file() and
truncate_pagecache(inode, 0). With some printks, we can see a clear
time gap (>2 second )  between collapse_file() finishes, and
truncate_pagecache() (which crashes soon). Therefore, my earlier
suggestion that adds deny_write_access() to collapse_file() does NOT
work.

The crash is actually caused by concurrent truncate_pagecache(inode, 0).
If I change the number of write thread in stress_madvise_dso.c to one,
(IOW, one thread_read and one thread_write), I cannot reproduce the
crash anymore.

I think this means we cannot fix this issue in collapse_file(), because it
finishes long before the crash.

Thanks,
Song