RE: [PATCH v4 1/2] mm: migration: fix the FOLL_GET failure on following huge page

[Wang, Haiyue]

> -----Original Message-----
> From: Alistair Popple <apopple@xxxxxxxxxx>
> Sent: Monday, August 15, 2022 13:35
> To: linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Wang, Haiyue <haiyue.wang@xxxxxxxxx>
> Cc: akpm@xxxxxxxxxxxxxxxxxxxx; david@xxxxxxxxxx; linmiaohe@xxxxxxxxxx; Huang, Ying
> <ying.huang@xxxxxxxxx>; songmuchun@xxxxxxxxxxxxx; naoya.horiguchi@xxxxxxxxx; alex.sierra@xxxxxxx
> Subject: Re: [PATCH v4 1/2] mm: migration: fix the FOLL_GET failure on following huge page
> 
> On Monday, 15 August 2022 3:20:28 PM AEST Wang, Haiyue wrote:
> > > -----Original Message-----
> > > From: Alistair Popple <apopple@xxxxxxxxxx>
> > > Sent: Monday, August 15, 2022 13:17
> > > To: linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Wang, Haiyue
> <haiyue.wang@xxxxxxxxx>
> > > Cc: akpm@xxxxxxxxxxxxxxxxxxxx; david@xxxxxxxxxx; linmiaohe@xxxxxxxxxx;
> Huang, Ying
> > > <ying.huang@xxxxxxxxx>; songmuchun@xxxxxxxxxxxxx;
> naoya.horiguchi@xxxxxxxxx; alex.sierra@xxxxxxx
> > > Subject: Re: [PATCH v4 1/2] mm: migration: fix the FOLL_GET failure on
> following huge page
> > >
> > > On Monday, 15 August 2022 2:40:48 PM AEST Wang, Haiyue wrote:
> > > > > -----Original Message-----
> > > > > From: Alistair Popple <apopple@xxxxxxxxxx>
> > > > > Sent: Monday, August 15, 2022 12:29
> > > > > To: linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Wang, Haiyue
> > > <haiyue.wang@xxxxxxxxx>
> > > > > Cc: akpm@xxxxxxxxxxxxxxxxxxxx; david@xxxxxxxxxx; linmiaohe@xxxxxxxxxx;
> > > Huang, Ying
> > > > > <ying.huang@xxxxxxxxx>; songmuchun@xxxxxxxxxxxxx;
> > > naoya.horiguchi@xxxxxxxxx; alex.sierra@xxxxxxx; Wang,
> > > > > Haiyue <haiyue.wang@xxxxxxxxx>
> > > > > Subject: Re: [PATCH v4 1/2] mm: migration: fix the FOLL_GET failure on
> > > following huge page
> > > > >
> > > > > On Monday, 15 August 2022 11:59:08 AM AEST Haiyue Wang 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.
> > > > >
> > > > > I think you should be explicit in the commit message about which
> functions
> > > do
> > > > > not support FOLL_GET as it's not obvious what support needs to be
> added
> > > before
> > > > > this fix can be reverted.
> > > >
> > > > Yes, make sense, will add them in new patch.
> > >
> > > Actually while you're at it I think it would be good to include a
> description
> > > of the impact of this failure in the commit message. Ie. You're answer to:
> > >
> > > > What are the user-visible runtime effects of this bug?
> > >
> > > As it documents what should be tested if this fix does actually ever get
> > > reverted.
> >
> > An short example *.c code to capture the bug in commit message ?
> 
> That's probably overkill. Just being a bit more explicit about the
> circumstances in which sys_move_pages() actually fails would be good. Eg.
> something like this:
> 
> "Without this sys_move_pages() will return -ENOENT for 1GB huge page
> memory map when dumping the page node information for nodes != NULL"

OK, understood, will try to add more information about the error. ;-)

> 
> > >
> > > > >
> > > > > Thanks.
> > > > >
> > > > >  - Alistair
> > > > >
> > > > > > 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.
> > > > > >
> > > > > > Fixes: 4cd614841c06 ("mm: migration: fix possible
> do_pages_stat_array
> > > racing
> > > > > with memory offline")
> > > > > > Signed-off-by: Haiyue Wang <haiyue.wang@xxxxxxxxx>
> > > > > > ---
> > > > > >  mm/migrate.c | 10 ++++++++--
> > > > > >  1 file changed, 8 insertions(+), 2 deletions(-)
> > > > > >
> > > > > > diff --git a/mm/migrate.c b/mm/migrate.c
> > > > > > index 6a1597c92261..581dfaad9257 100644
> > > > > > --- a/mm/migrate.c
> > > > > > +++ b/mm/migrate.c
> > > > > > @@ -1848,6 +1848,7 @@ static void do_pages_stat_array(struct
> mm_struct
> > > *mm,
> > > > > unsigned long nr_pages,
> > > > > >
> > > > > >  	for (i = 0; i < nr_pages; i++) {
> > > > > >  		unsigned long addr = (unsigned long)(*pages);
> > > > > > +		unsigned int foll_flags = FOLL_DUMP;
> > > > > >  		struct vm_area_struct *vma;
> > > > > >  		struct page *page;
> > > > > >  		int err = -EFAULT;
> > > > > > @@ -1856,8 +1857,12 @@ static void do_pages_stat_array(struct
> mm_struct
> > > *mm,
> > > > > unsigned long nr_pages,
> > > > > >  		if (!vma)
> > > > > >  			goto set_status;
> > > > > >
> > > > > > +		/* Not all huge page follow APIs support 'FOLL_GET'
> */
> > > > > > +		if (!is_vm_hugetlb_page(vma))
> > > > > > +			foll_flags |= FOLL_GET;
> > > > > > +
> > > > > >  		/* FOLL_DUMP to ignore special (like zero) pages */
> > > > > > -		page = follow_page(vma, addr, FOLL_GET | FOLL_DUMP);
> > > > > > +		page = follow_page(vma, addr, foll_flags);
> > > > > >
> > > > > >  		err = PTR_ERR(page);
> > > > > >  		if (IS_ERR(page))
> > > > > > @@ -1865,7 +1870,8 @@ static void do_pages_stat_array(struct
> mm_struct
> > > *mm,
> > > > > unsigned long nr_pages,
> > > > > >
> > > > > >  		if (page && !is_zone_device_page(page)) {
> > > > > >  			err = page_to_nid(page);
> > > > > > -			put_page(page);
> > > > > > +			if (foll_flags & FOLL_GET)
> > > > > > +				put_page(page);
> > > > > >  		} else {
> > > > > >  			err = -ENOENT;
> > > > > >  		}
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