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

[Song Liu <songliubraving@fb.com>]

> On Nov 7, 2019, at 7:53 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> On Tue, 5 Nov 2019 22:09:29 -0800 Song Liu <songliubraving@fb.com> 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 ;))

This is new feature (and new bug :( ) in 5.4. So no need to back port.

Thanks,
Song