On Fri, 18 Oct 2019 11:03:45 -0700 Song Liu <songliubraving@xxxxxx> wrote:
> In collapse_file(), after locking the page, it is necessary to recheck
> that the page is up-to-date. Add PageUptodate() check for both shmem THP
> and file THP.
>
> Current khugepaged should not try to collapse dirty file THP, because it
> is limited to read only text. Add a PageDirty check and warning for file
> THP. This is added after page_mapping() check, because if the page is
> truncated, it might be dirty.
When fixing a bug, please always fully describe the end-user visible
effects of that bug. This is vital information for people who are
considering the fix for backporting.
I'm suspecting that you've found a race condition which can trigger a
VM_BUG_ON_PAGE(), which is rather serious. But that was just a wild
guess. Please don't make us wildly guess :(
The old code looked rather alarming:
} else if (!PageUptodate(page)) {
xas_unlock_irq(&xas);
wait_on_page_locked(page);
if (!trylock_page(page)) {
result = SCAN_PAGE_LOCK;
goto xa_unlocked;
}
get_page(page);
We don't have a ref on that page. After we've released the xarray lock
we have no business playing with *page at all, correct?
