On 23 Oct 2023, at 17:10, Hugh Dickins wrote: > 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. Got it. Thanks for the explanation. I should have finished the whole series. -- Best Regards, Yan, Zi
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