Re: [External] Re: [PATCH] hugetlb: fix uninitialized subpool pointer

[Muchun Song]

On Wed, Feb 24, 2021 at 5:43 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Tue 23-02-21 13:55:44, Mike Kravetz wrote:
> > Gerald Schaefer reported a panic on s390 in hugepage_subpool_put_pages()
> > with linux-next 5.12.0-20210222.
> > Call trace:
> >   hugepage_subpool_put_pages.part.0+0x2c/0x138
> >   __free_huge_page+0xce/0x310
> >   alloc_pool_huge_page+0x102/0x120
> >   set_max_huge_pages+0x13e/0x350
> >   hugetlb_sysctl_handler_common+0xd8/0x110
> >   hugetlb_sysctl_handler+0x48/0x58
> >   proc_sys_call_handler+0x138/0x238
> >   new_sync_write+0x10e/0x198
> >   vfs_write.part.0+0x12c/0x238
> >   ksys_write+0x68/0xf8
> >   do_syscall+0x82/0xd0
> >   __do_syscall+0xb4/0xc8
> >   system_call+0x72/0x98
> >
> > This is a result of the change which moved the hugetlb page subpool
> > pointer from page->private to page[1]->private.  When new pages are
> > allocated from the buddy allocator, the private field of the head
> > page will be cleared, but the private field of subpages is not modified.
> > Therefore, old values may remain.
>
> Very interesting. I have expected that the page->private would be in a
> reasonable state when allocated. On the other hand hugetlb doesn't do
> __GFP_COMP so tail pages are not initialized by the allocator.

It seems that the buddy allocator does not initialize the private field
of the tail page even when we specify __GFP_COMP.


>
> > Fix by initializing hugetlb page subpool pointer in prep_new_huge_page().
> >
> > Fixes: f1280272ae4d ("hugetlb: use page.private for hugetlb specific page flags")
>
> This is not a stable sha to refer to as it comes from linux next.
>
> > Reported-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
> > Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>
> I think this would be worth a separate patch rather than having it
> folded into the original patch. Thi is really subtle.
>
> > ---
> >  mm/hugetlb.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index c232cb67dda2..7ae5c18c98a7 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1465,6 +1465,7 @@ static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
> >  {
> >       INIT_LIST_HEAD(&page->lru);
> >       set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> > +     hugetlb_set_page_subpool(page, NULL);
> >       set_hugetlb_cgroup(page, NULL);
> >       set_hugetlb_cgroup_rsvd(page, NULL);
> >       spin_lock(&hugetlb_lock);
> > --
> > 2.29.2
> >
>
> --
> Michal Hocko
> SUSE Labs