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Message-ID: <87mtc4vr64.fsf@nvdebian.thelocal>
Date: Tue, 16 Aug 2022 12:45:23 +1000
From: Alistair Popple <apopple@...dia.com>
To: "Wang, Haiyue" <haiyue.wang@...el.com>
Cc: "linux-mm@...ck.org" <linux-mm@...ck.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
"david@...hat.com" <david@...hat.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>,
Felix Kuehling <Felix.Kuehling@....com>
Subject: Re: [PATCH v5 2/2] mm: fix the handling Non-LRU pages returned by
follow_page
"Wang, Haiyue" <haiyue.wang@...el.com> writes:
>> -----Original Message-----
>> From: Alistair Popple <apopple@...dia.com>
>> Sent: Tuesday, August 16, 2022 08:01
>> To: Wang, Haiyue <haiyue.wang@...el.com>
>> Cc: linux-mm@...ck.org; linux-kernel@...r.kernel.org; akpm@...ux-foundation.org; david@...hat.com;
>> linmiaohe@...wei.com; Huang, Ying <ying.huang@...el.com>; songmuchun@...edance.com;
>> naoya.horiguchi@...ux.dev; alex.sierra@....com; Felix Kuehling <Felix.Kuehling@....com>
>> Subject: Re: [PATCH v5 2/2] mm: fix the handling Non-LRU pages returned by follow_page
>>
>>
>> Haiyue Wang <haiyue.wang@...el.com> writes:
>>
>> > 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 this
>> > 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@...el.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, unsigned long vaddr_start,
>> > /* FOLL_DUMP to ignore special (like zero) pages */
>> > page = 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, unsigned long addr)
>> > cond_resched();
>> > page = 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 = handle_mm_fault(vma, addr,
>> > @@ -560,12 +560,15 @@ static struct page *get_mergeable_page(struct rmap_item *rmap_item)
>> > goto out;
>> >
>> > page = 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))
>>
>> 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.
>>
>
> The 'page' for scan_get_next_rmap_item() is from 'vma' which is NOT MERGEABLE:
> for (; vma; vma = vma->vm_next) {
> if (!(vma->vm_flags & VM_MERGEABLE))
> continue;
>
> The 'page' for get_mergeable_page() is from 'vma' which is MERGEABLE by 'find_mergeable_vma()'
Oh, ok. I'm actually not too familiar with KSM but I think I follow so
if you think we need to keep the check by all means do so.
> So they may be different, and the unstable_tree_search_insert() shows the logical:
>
> 'page' vs 'tree_page':
>
> tree_page = get_mergeable_page(tree_rmap_item);
> if (!tree_page)
> return NULL;
>
> /*
> * Don't substitute a ksm page for a forked page.
> */
> if (page == tree_page) {
> put_page(tree_page);
> return NULL;
> }
>
> ret = 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 = NULL;
>> > @@ -2308,11 +2311,13 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page)
>> > if (ksm_test_exit(mm))
>> > break;
>> > *page = 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 += 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 += 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_struct *mm, unsigned long addr,
>> > goto out;
>> >
>> > err = -ENOENT;
>> > - if (!page || is_zone_device_page(page))
>> > + if (!page)
>> > goto out;
>> >
>> > + if (is_zone_device_page(page))
>> > + goto out_putpage;
>> > +
>> > err = 0;
>> > if (page_to_nid(page) == 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 = page_to_nid(page);
>> > + if (page) {
>> > + err = !is_zone_device_page(page) ? page_to_nid(page)
>> > + : -ENOENT;
>>
>> 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.
Thanks.
>>
>> - if (page && !is_zone_device_page(page)) {
>> - err = page_to_nid(page);
>> - if (foll_flags & FOLL_GET)
>> - put_page(page);
>> - } else {
>> + if (!page) {
>> err = -ENOENT;
>> + goto set_status;
>> }
>> +
>> + if (is_zone_device_page(page))
>> + err = -ENOENT;
>> + else
>> + err = page_to_nid_page(page);
>> +
>> + if (foll_flags & FOLL_GET)
>> + put_page(page);
>>
>> Thanks.
>>
>> - Alistair
>>
>> > if (foll_flags & FOLL_GET)
>> > put_page(page);
>> > } else {
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