> On Nov 7, 2019, at 7:53 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > 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 ;)) This is new feature (and new bug :( ) in 5.4. So no need to back port. Thanks, Song
