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Message-ID: <aa5f235e-6449-1531-1355-6974f3d38740@redhat.com>
Date: Mon, 20 Jan 2020 15:13:54 +0100
From: David Hildenbrand <david@...hat.com>
To: Qian Cai <cai@....pw>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm v2] mm/page_isolation: fix potential warning from user
On 20.01.20 15:11, Qian Cai wrote:
>
>
>> On Jan 20, 2020, at 9:01 AM, David Hildenbrand <david@...hat.com> wrote:
>>
>> On 20.01.20 14:56, Qian Cai wrote:
>>>
>>>
>>>> On Jan 20, 2020, at 8:38 AM, David Hildenbrand <david@...hat.com> wrote:
>>>>
>>>> On 20.01.20 14:30, David Hildenbrand wrote:
>>>>> On 20.01.20 14:19, Qian Cai wrote:
>>>>>> It makes sense to call the WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE)
>>>>>> from start_isolate_page_range(), but should avoid triggering it from
>>>>>> userspace, i.e, from is_mem_section_removable() because it could be a
>>>>>> DoS if warn_on_panic is set.
>>>>>>
>>>>>> While at it, simplify the code a bit by removing an unnecessary jump
>>>>>> label and a local variable, so set_migratetype_isolate() could really
>>>>>> return a bool.
>>>>>>
>>>>>> Suggested-by: Michal Hocko <mhocko@...nel.org>
>>>>>> Signed-off-by: Qian Cai <cai@....pw>
>>>>>> ---
>>>>>>
>>>>>> v2: Improve the commit log.
>>>>>> Warn for all start_isolate_page_range() users not just offlining.
>>>>>>
>>>>>> mm/page_alloc.c | 11 ++++-------
>>>>>> mm/page_isolation.c | 30 +++++++++++++++++-------------
>>>>>> 2 files changed, 21 insertions(+), 20 deletions(-)
>>>>>>
>>>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>>>> index 621716a25639..3c4eb750a199 100644
>>>>>> --- a/mm/page_alloc.c
>>>>>> +++ b/mm/page_alloc.c
>>>>>> @@ -8231,7 +8231,7 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>>>>>> if (is_migrate_cma(migratetype))
>>>>>> return NULL;
>>>>>>
>>>>>> - goto unmovable;
>>>>>> + return page;
>>>>>> }
>>>>>>
>>>>>> for (; iter < pageblock_nr_pages; iter++) {
>>>>>> @@ -8241,7 +8241,7 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>>>>>> page = pfn_to_page(pfn + iter);
>>>>>>
>>>>>> if (PageReserved(page))
>>>>>> - goto unmovable;
>>>>>> + return page;
>>>>>>
>>>>>> /*
>>>>>> * If the zone is movable and we have ruled out all reserved
>>>>>> @@ -8261,7 +8261,7 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>>>>>> unsigned int skip_pages;
>>>>>>
>>>>>> if (!hugepage_migration_supported(page_hstate(head)))
>>>>>> - goto unmovable;
>>>>>> + return page;
>>>>>>
>>>>>> skip_pages = compound_nr(head) - (page - head);
>>>>>> iter += skip_pages - 1;
>>>>>> @@ -8303,12 +8303,9 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>>>>>> * is set to both of a memory hole page and a _used_ kernel
>>>>>> * page at boot.
>>>>>> */
>>>>>> - goto unmovable;
>>>>>> + return page;
>>>>>> }
>>>>>> return NULL;
>>>>>> -unmovable:
>>>>>> - WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);
>>>>>> - return pfn_to_page(pfn + iter);
>>>>>> }
>>>>>>
>>>>>> #ifdef CONFIG_CONTIG_ALLOC
>>>>>> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
>>>>>> index e70586523ca3..31f5516f5d54 100644
>>>>>> --- a/mm/page_isolation.c
>>>>>> +++ b/mm/page_isolation.c
>>>>>> @@ -15,12 +15,12 @@
>>>>>> #define CREATE_TRACE_POINTS
>>>>>> #include <trace/events/page_isolation.h>
>>>>>>
>>>>>> -static int set_migratetype_isolate(struct page *page, int migratetype, int isol_flags)
>>>>>> +static bool set_migratetype_isolate(struct page *page, int migratetype,
>>>>>> + int isol_flags)
>>>>>
>>>>> Why this change?
>>>>>
>>>>>> {
>>>>>> - struct page *unmovable = NULL;
>>>>>> + struct page *unmovable = ERR_PTR(-EBUSY);
>>>>>
>>>>> Also, why this change?
>>>>>
>>>>>> struct zone *zone;
>>>>>> unsigned long flags;
>>>>>> - int ret = -EBUSY;
>>>>>>
>>>>>> zone = page_zone(page);
>>>>>>
>>>>>> @@ -49,21 +49,25 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
>>>>>> NULL);
>>>>>>
>>>>>> __mod_zone_freepage_state(zone, -nr_pages, mt);
>>>>>> - ret = 0;
>>>>>> }
>>>>>>
>>>>>> out:
>>>>>> spin_unlock_irqrestore(&zone->lock, flags);
>>>>>> - if (!ret)
>>>>>> +
>>>>>> + if (!unmovable) {
>>>>>> drain_all_pages(zone);
>>>>>> - else if ((isol_flags & REPORT_FAILURE) && unmovable)
>>>>>> - /*
>>>>>> - * printk() with zone->lock held will guarantee to trigger a
>>>>>> - * lockdep splat, so defer it here.
>>>>>> - */
>>>>>> - dump_page(unmovable, "unmovable page");
>>>>>> -
>>>>>> - return ret;
>>>>>> + } else {
>>>>>> + WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);
>>>>>> +
>>>>>> + if ((isol_flags & REPORT_FAILURE) && !IS_ERR(unmovable))
>>>>>> + /*
>>>>>
>>>>> Why this change? (!IS_ERR)
>>>>>
>>>>>
>>>>> Some things here look unrelated - or I am missing something :)
>>>>>
>>>>
>>>> FWIW, I'd prefer this change without any such cleanups (e.g., I don't
>>>> like returning a bool from this function and the IS_ERR handling, makes
>>>> the function harder to read than before)
>>>
>>> What is Michal or Andrew’s opinion? BTW, a bonus point to return a bool
>>> is that it helps the code robustness in general, as UBSAN will be able to
>>> catch any abuse.
>>>
>>
>> A return type of bool on a function that does not test a property
>> ("has_...", "is"...") is IMHO confusing.
>
> That is fine. It could be renamed to set_migratetype_is_isolate() or
> is_set_migratetype_isolate() which seems pretty minor because we
> have no consistency in the naming of this in linux kernel at all, i.e.,
> many existing bool function names without those test of properties.
It does not query a property, so "is_set_migratetype_isolate()" is plain
wrong.
Anyhow, Michal does not seem to care.
--
Thanks,
David / dhildenb
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