linux-kernel - RE: [PATCH v6 1/2] mm: migration: fix the FOLL

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Date:   Wed, 17 Aug 2022 03:31:37 +0000
From:   "Wang, Haiyue" <haiyue.wang@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "david@...hat.com" <david@...hat.com>,
        "apopple@...dia.com" <apopple@...dia.com>,
        "linmiaohe@...wei.com" <linmiaohe@...wei.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "songmuchun@...edance.com" <songmuchun@...edance.com>,
        "naoya.horiguchi@...ux.dev" <naoya.horiguchi@...ux.dev>,
        "alex.sierra@....com" <alex.sierra@....com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: RE: [PATCH v6 1/2] mm: migration: fix the FOLL_GET failure on
 following huge page

> -----Original Message-----
> From: Andrew Morton <akpm@...ux-foundation.org>
> Sent: Wednesday, August 17, 2022 08:59
> To: Wang, Haiyue <haiyue.wang@...el.com>
> Cc: linux-mm@...ck.org; linux-kernel@...r.kernel.org; david@...hat.com; apopple@...dia.com;
> linmiaohe@...wei.com; Huang, Ying <ying.huang@...el.com>; songmuchun@...edance.com;
> naoya.horiguchi@...ux.dev; alex.sierra@....com; Heiko Carstens <hca@...ux.ibm.com>; Vasily Gorbik
> <gor@...ux.ibm.com>; Alexander Gordeev <agordeev@...ux.ibm.com>; Christian Borntraeger
> <borntraeger@...ux.ibm.com>; Sven Schnelle <svens@...ux.ibm.com>; Mike Kravetz
> <mike.kravetz@...cle.com>
> Subject: Re: [PATCH v6 1/2] mm: migration: fix the FOLL_GET failure on following huge page
> 
> On Tue, 16 Aug 2022 10:21:00 +0800 Haiyue Wang <haiyue.wang@...el.com> wrote:
> 
> > Not all huge page APIs support FOLL_GET option, so move_pages() syscall
> > will fail to get the page node information for some huge pages.
> >
> > Like x86 on linux 5.19 with 1GB huge page API follow_huge_pud(), it will
> > return NULL page for FOLL_GET when calling move_pages() syscall with the
> > NULL 'nodes' parameter, the 'status' parameter has '-2' error in array.
> >
> > Note: follow_huge_pud() now supports FOLL_GET in linux 6.0.
> >       Link: https://lore.kernel.org/all/20220714042420.1847125-3-naoya.horiguchi@linux.dev
> >
> > But these huge page APIs don't support FOLL_GET:
> >   1. follow_huge_pud() in arch/s390/mm/hugetlbpage.c
> 
> Let's tell the s390 maintainers.
> 
> >   2. follow_huge_addr() in arch/ia64/mm/hugetlbpage.c
> >      It will cause WARN_ON_ONCE for FOLL_GET.
> 
> ia64 doesn't have maintainers :( Can we come up with something local to
> arch/ia64 for this?

The 'follow_huge_addr' itself just has interest on "FOLL_WRITE"
struct page *
follow_huge_addr(struct mm_struct *mm, unsigned long address,
			      int write)

And arch/ia64 defines this function 17 years ago ...

But I found that "WARN_ON_ONCE for FOLL_GET" was introduced on 2005-10-29
by commit:

[PATCH] mm: follow_page with inner ptlock

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=deceb6cd17e6dfafe4c4f81b1b4153bc41b2cb70

-	page = follow_huge_addr(mm, address, write);
-	if (! IS_ERR(page))
-		return page;
+	page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
+	if (!IS_ERR(page)) {
+		BUG_ON(flags & FOLL_GET);
+		goto out;
+	}

> 
> >   3. follow_huge_pgd() in mm/hugetlb.c
> 
> Hi, Mike.
> 


> >  		}
> 
> I would be better to fix this for real at those three client code sites?

Then 5.19 will break for a while to wait for the final BIG patch ?