On Mon 19-11-18 12:34:09, Hugh Dickins wrote: > On Mon, 19 Nov 2018, Michal Hocko wrote: > > On Mon 19-11-18 15:10:16, Michal Hocko wrote: > > [...] > > > In other words. Why cannot we do the following? > > > > Baoquan, this is certainly not the right fix but I would be really > > curious whether it makes the problem go away. > > > > > diff --git a/mm/migrate.c b/mm/migrate.c > > > index f7e4bfdc13b7..7ccab29bcf9a 100644 > > > --- a/mm/migrate.c > > > +++ b/mm/migrate.c > > > @@ -324,19 +324,9 @@ void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep, > > > goto out; > > > > > > page = migration_entry_to_page(entry); > > > - > > > - /* > > > - * Once page cache replacement of page migration started, page_count > > > - * *must* be zero. And, we don't want to call wait_on_page_locked() > > > - * against a page without get_page(). > > > - * So, we use get_page_unless_zero(), here. Even failed, page fault > > > - * will occur again. > > > - */ > > > - if (!get_page_unless_zero(page)) > > > - goto out; > > > pte_unmap_unlock(ptep, ptl); > > > - wait_on_page_locked(page); > > > - put_page(page); > > > + page_lock(page); > > > + page_unlock(page); > > > return; > > > out: > > > pte_unmap_unlock(ptep, ptl); > > Thanks for Cc'ing me. I did mention precisely this issue two or three > times at LSF/MM this year, and claimed then that I would post the fix. I've had a recollection about some issue I just couldn't remember what was it exactly. Tried to make Vlastimil to remember but failed there as well ;) > I'm glad that I delayed, what I had then (migration_waitqueue instead > of using page_waitqueue) was not wrong, but what I've been using the > last couple of months is rather better (and can be put to use to solve > similar problems in collapsing pages on huge tmpfs. but we don't need > to get into that at this time): put_and_wait_on_page_locked(). > > What I have not yet done is verify it on latest kernel, and research > the interested Cc list (Linus and Tim Chen come immediately to mind), > and write the commit comment. I have some testing to do on the latest > kernel today, so I'll throw put_and_wait_on_page_locked() in too, > and post tomorrow I hope. Cool, it seems that Baoquan has a reliable test case to trigger the pathological case. -- Michal Hocko SUSE Labs
