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From:   "Wang, Haiyue" <haiyue.wang@intel.com>
To:     Alistair Popple <apopple@nvidia.com>
CC:     "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "david@redhat.com" <david@redhat.com>,
        "linmiaohe@huawei.com" <linmiaohe@huawei.com>,
        "Huang, Ying" <ying.huang@intel.com>,
        "songmuchun@bytedance.com" <songmuchun@bytedance.com>,
        "naoya.horiguchi@linux.dev" <naoya.horiguchi@linux.dev>,
        "alex.sierra@amd.com" <alex.sierra@amd.com>,
        Felix Kuehling <Felix.Kuehling@amd.com>
Subject: RE: [PATCH v5 2/2] mm: fix the handling Non-LRU pages returned by
 follow_page
Thread-Topic: [PATCH v5 2/2] mm: fix the handling Non-LRU pages returned by
 follow_page
Thread-Index: AQHYsHUAsdqcAiEDdEqE2PI2IKO5la2wpY+AgAANwTA=
Date:   Tue, 16 Aug 2022 01:12:23 +0000
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> -----Original Message-----
> From: Alistair Popple <apopple@nvidia.com>
> Sent: Tuesday, August 16, 2022 08:01
> To: Wang, Haiyue <haiyue.wang@intel.com>
> Cc: linux-mm@kvack.org; linux-kernel@vger.kernel.org; akpm@linux-foundati=
on.org; david@redhat.com;
> linmiaohe@huawei.com; Huang, Ying <ying.huang@intel.com>; songmuchun@byte=
dance.com;
> naoya.horiguchi@linux.dev; alex.sierra@amd.com; Felix Kuehling <Felix.Kue=
hling@amd.com>
> Subject: Re: [PATCH v5 2/2] mm: fix the handling Non-LRU pages returned b=
y follow_page
>=20
>=20
> Haiyue Wang <haiyue.wang@intel.com> writes:
>=20
> > The handling Non-LRU pages returned by follow_page() jumps directly, it
> > doesn't call put_page() to handle the reference count, since 'FOLL_GET'
> > flag for follow_page() has get_page() called. Fix the zone device page
> > check by handling the page reference count correctly before returning.
> >
> > And as David reviewed, "device pages are never PageKsm pages". Drop thi=
s
> > zone device page check for break_ksm().
> >
> > Fixes: 3218f8712d6b ("mm: handling Non-LRU pages returned by vm_normal_=
pages")
> > Signed-off-by: Haiyue Wang <haiyue.wang@intel.com>
> > ---
> >  mm/huge_memory.c |  4 ++--
> >  mm/ksm.c         | 12 +++++++++---
> >  mm/migrate.c     | 10 +++++++---
> >  3 files changed, 18 insertions(+), 8 deletions(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 8a7c1b344abe..b2ba17c3dcd7 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2963,10 +2963,10 @@ static int split_huge_pages_pid(int pid, unsign=
ed long vaddr_start,
> >  		/* FOLL_DUMP to ignore special (like zero) pages */
> >  		page =3D follow_page(vma, addr, FOLL_GET | FOLL_DUMP);
> >
> > -		if (IS_ERR_OR_NULL(page) || is_zone_device_page(page))
> > +		if (IS_ERR_OR_NULL(page))
> >  			continue;
> >
> > -		if (!is_transparent_hugepage(page))
> > +		if (is_zone_device_page(page) || !is_transparent_hugepage(page))
> >  			goto next;
> >
> >  		total++;
> > diff --git a/mm/ksm.c b/mm/ksm.c
> > index 42ab153335a2..e26f57fc1f0e 100644
> > --- a/mm/ksm.c
> > +++ b/mm/ksm.c
> > @@ -475,7 +475,7 @@ static int break_ksm(struct vm_area_struct *vma, un=
signed long addr)
> >  		cond_resched();
> >  		page =3D follow_page(vma, addr,
> >  				FOLL_GET | FOLL_MIGRATION | FOLL_REMOTE);
> > -		if (IS_ERR_OR_NULL(page) || is_zone_device_page(page))
> > +		if (IS_ERR_OR_NULL(page))
> >  			break;
> >  		if (PageKsm(page))
> >  			ret =3D handle_mm_fault(vma, addr,
> > @@ -560,12 +560,15 @@ static struct page *get_mergeable_page(struct rma=
p_item *rmap_item)
> >  		goto out;
> >
> >  	page =3D follow_page(vma, addr, FOLL_GET);
> > -	if (IS_ERR_OR_NULL(page) || is_zone_device_page(page))
> > +	if (IS_ERR_OR_NULL(page))
> >  		goto out;
> > +	if (is_zone_device_page(page))
>=20
> Same as for break_ksm() I think we should be able to drop the
> is_zone_device_page() check here because scan_get_next_rmap_item()
> already filters out zone device pages.
>=20

The 'page' for scan_get_next_rmap_item() is from 'vma' which is NOT MERGEAB=
LE:
	for (; vma; vma =3D vma->vm_next) {
		if (!(vma->vm_flags & VM_MERGEABLE))
			continue;

The 'page' for get_mergeable_page() is from 'vma' which is MERGEABLE by 'fi=
nd_mergeable_vma()'

So they may be different, and the unstable_tree_search_insert() shows the l=
ogical:

 'page' vs 'tree_page':

		tree_page =3D get_mergeable_page(tree_rmap_item);
		if (!tree_page)
			return NULL;

		/*
		 * Don't substitute a ksm page for a forked page.
		 */
		if (page =3D=3D tree_page) {
			put_page(tree_page);
			return NULL;
		}

		ret =3D memcmp_pages(page, tree_page);


> > +		goto out_putpage;
> >  	if (PageAnon(page)) {
> >  		flush_anon_page(vma, page, addr);
> >  		flush_dcache_page(page);
> >  	} else {
> > +out_putpage:
> >  		put_page(page);
> >  out:
> >  		page =3D NULL;
> > @@ -2308,11 +2311,13 @@ static struct rmap_item *scan_get_next_rmap_ite=
m(struct page **page)
> >  			if (ksm_test_exit(mm))
> >  				break;
> >  			*page =3D follow_page(vma, ksm_scan.address, FOLL_GET);
> > -			if (IS_ERR_OR_NULL(*page) || is_zone_device_page(*page)) {
> > +			if (IS_ERR_OR_NULL(*page)) {
> >  				ksm_scan.address +=3D PAGE_SIZE;
> >  				cond_resched();
> >  				continue;
> >  			}
> > +			if (is_zone_device_page(*page))
> > +				goto next_page;
> >  			if (PageAnon(*page)) {
> >  				flush_anon_page(vma, *page, ksm_scan.address);
> >  				flush_dcache_page(*page);
> > @@ -2327,6 +2332,7 @@ static struct rmap_item *scan_get_next_rmap_item(=
struct page **page)
> >  				mmap_read_unlock(mm);
> >  				return rmap_item;
> >  			}
> > +next_page:
> >  			put_page(*page);
> >  			ksm_scan.address +=3D PAGE_SIZE;
> >  			cond_resched();
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 581dfaad9257..fee12cd2f294 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -1672,9 +1672,12 @@ static int add_page_for_migration(struct mm_stru=
ct *mm, unsigned long addr,
> >  		goto out;
> >
> >  	err =3D -ENOENT;
> > -	if (!page || is_zone_device_page(page))
> > +	if (!page)
> >  		goto out;
> >
> > +	if (is_zone_device_page(page))
> > +		goto out_putpage;
> > +
> >  	err =3D 0;
> >  	if (page_to_nid(page) =3D=3D node)
> >  		goto out_putpage;
> > @@ -1868,8 +1871,9 @@ static void do_pages_stat_array(struct mm_struct =
*mm, unsigned long nr_pages,
> >  		if (IS_ERR(page))
> >  			goto set_status;
> >
> > -		if (page && !is_zone_device_page(page)) {
> > -			err =3D page_to_nid(page);
> > +		if (page) {
> > +			err =3D !is_zone_device_page(page) ? page_to_nid(page)
> > +							 : -ENOENT;
>=20
> Can we remove the multiple layers of conditionals here? Something like
> this is cleaner and easier to understand IMHO:

OK, I will try it in new patch.

>=20
> -               if (page && !is_zone_device_page(page)) {
> -                       err =3D page_to_nid(page);
> -                       if (foll_flags & FOLL_GET)
> -                               put_page(page);
> -               } else {
> +               if (!page) {
>                         err =3D -ENOENT;
> +                       goto set_status;
>                 }
> +
> +               if (is_zone_device_page(page))
> +                       err =3D -ENOENT;
> +               else
> +                       err =3D page_to_nid_page(page);
> +
> +               if (foll_flags & FOLL_GET)
> +                       put_page(page);
>=20
> Thanks.
>=20
>  - Alistair
>=20
> >  			if (foll_flags & FOLL_GET)
> >  				put_page(page);
> >  		} else {