On Mon, 23 Oct 2023, Zi Yan wrote: > On 19 Oct 2023, at 16:39, Hugh Dickins wrote: > > @@ -1170,6 +1170,15 @@ static struct folio *new_folio(struct folio *src, unsigned long start) > > break; > > } > > > > + /* > > + * __get_vma_policy() now expects a genuine non-NULL vma. Return NULL > > + * when the page can no longer be located in a vma: that is not ideal > > + * (migrate_pages() will give up early, presuming ENOMEM), but good > > + * enough to avoid a crash by syzkaller or concurrent holepunch. > > + */ > > + if (!vma) > > + return NULL; > > + > > How often would this happen? I just want to point out that ENOMEM can cause > src THPs or large folios to be split by migrate_pages(). The only case I know of it happening was when a file was mapped, then that file truncated (cutting out the source page) before migrate_pages(&pagelist) reached it - rather a syzbotty thing to do, not of great reallife concern. I won't assert that's the only way: I've a ghost of a memory of another way, that I can't quite resurface, from a long-ago version of queue_pages_range(). But in the end just didn't care enough about it, because this is really just to save a bisection point from crashing - the possibility goes away in the 11/12 commit which follows this one, which takes VMA out of it altogether. Hugh
