RE: [PATCH v1] mm: migration: fix the FOLL_GET failure on following huge page

[Wang, Haiyue]

> -----Original Message-----
> From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Sent: Sunday, August 14, 2022 07:29
> To: Wang, Haiyue <haiyue.wang@xxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; david@xxxxxxxxxx; linmiaohe@xxxxxxxxxx; Huang,
> Ying <ying.huang@xxxxxxxxx>; songmuchun@xxxxxxxxxxxxx; naoya.horiguchi@xxxxxxxxx
> Subject: Re: [PATCH v1] mm: migration: fix the FOLL_GET failure on following huge page
> 
> On Fri, 12 Aug 2022 16:49:21 +0800 Haiyue Wang <haiyue.wang@xxxxxxxxx> wrote:
> 
> > Not all huge page APIs support FOLL_GET option, so the __NR_move_pages
> > will fail to get the page node information for huge page.
> 
> Which ones need fixing?

1. 'follow_huge_pud' arch/s390/mm/hugetlbpage.c

2. 'follow_huge_addr' arch/ia64/mm/hugetlbpage.c

3. 'follow_huge_pgd' mm/hugetlb.c

And I found that only 'pud' and 'pmd' need to check 'is_vm_hugetlb_page' like:
pud_huge(*pud) && is_vm_hugetlb_page(vma)
pmd_huge(pmdval) && is_vm_hugetlb_page(vma)

So I'm not sure whether my patch can cover 2 & 3 for other huge page use cases
except by hugetlbfs.

> 
> What are the user-visible runtime effects of this bug?
> 

In my test, the '__NR_move_pages' system call will return '-2' for 1GB huge page
memory map when dump the page node information. [Test on linux-5.19 stable]

> Is a -stable backport warranted?

Yes.

Since the mainline has introduced the new patch:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3218f8712d6bb

The backported needs to rebase like for 5.19:

-		if (page && !is_zone_device_page(page)) {
+		if (page) {

> 
> > This is an temporary solution to mitigate the racing fix.
> >
> > After supporting follow huge page by FOLL_GET is done, this fix can be
> > reverted safely.
> >