Re: [PATCH v5 1/2] mm,thp: recheck each page before collapsing file THP

[Andrew Morton]

On Tue, 5 Nov 2019 22:09:29 -0800 Song Liu <songliubraving@xxxxxx> wrote:

> In collapse_file(), for !is_shmem case, current check cannot guarantee
> the locked page is up-to-date.  Specifically, xas_unlock_irq() should
> not be called before lock_page() and get_page(); and it is necessary to
> recheck PageUptodate() after locking the page.
> 
> With this bug and CONFIG_READ_ONLY_THP_FOR_FS=y, madvise(HUGE)'ed .text
> may contain corrupted data.  This is because khugepaged mistakenly
> collapses some not up-to-date sub pages into a huge page, and assumes
> the huge page is up-to-date.  This will NOT corrupt data in the disk,
> because the page is read-only and never written back.  Fix this by
> properly checking PageUptodate() after locking the page.  This check
> replaces "VM_BUG_ON_PAGE(!PageUptodate(page), page);".
> 
> Also, move PageDirty() check after locking the page.  Current
> khugepaged should not try to collapse dirty file THP, because it is
> limited to read-only .text. The only case we hit a dirty page here is
> when the page hasn't been written since write. Bail out and retry when
> this happens.

Incorrect data is pretty serious.  Should we backport this into -stable
kernels?

(I suspect I already asked this in response to earier versions, sorry ;))