Messages in this thread | | | Date | Fri, 18 Oct 2019 18:17:12 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH v3] mm,thp: recheck each page before collapsing file THP |
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On Fri, 18 Oct 2019 11:03:45 -0700 Song Liu <songliubraving@fb.com> 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?
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