Re: [PATCH v3] mm,thp: recheck each page before collapsing file THP

[Andrew Morton]

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?