When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
return false positive for PTE-mapped THP since page_mapcount() is used
to check if the THP is unmapped, but it just checks compound mapount and
head page's mapcount. If the THP is PTE-mapped and head page is not
mapped, it may return false positive.
Use total_mapcount() instead of page_mapcount() and do so for the
VM_BUG_ON_PAGE in split_huge_page_to_list as well.
[1] https://lore.kernel.org/linux-mm/[email protected]/
Signed-off-by: Yang Shi <[email protected]>
---
mm/huge_memory.c | 2 +-
mm/rmap.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 63ed6b25deaa..2122c3e853b9 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2718,7 +2718,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
}
unmap_page(head);
- VM_BUG_ON_PAGE(compound_mapcount(head), head);
+ VM_BUG_ON_PAGE(total_mapcount(head), head);
/* block interrupt reentry in xa_lock and spinlock */
local_irq_disable();
diff --git a/mm/rmap.c b/mm/rmap.c
index 693a610e181d..2e547378ab5f 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1777,7 +1777,7 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
else
rmap_walk(page, &rwc);
- return !page_mapcount(page) ? true : false;
+ return !total_mapcount(page) ? true : false;
}
/**
--
2.26.2
On 30 Apr 2021, at 17:07, Yang Shi wrote:
> When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
> return false positive for PTE-mapped THP since page_mapcount() is used
> to check if the THP is unmapped, but it just checks compound mapount and
> head page's mapcount. If the THP is PTE-mapped and head page is not
> mapped, it may return false positive.
>
> Use total_mapcount() instead of page_mapcount() and do so for the
> VM_BUG_ON_PAGE in split_huge_page_to_list as well.
>
> [1] https://lore.kernel.org/linux-mm/[email protected]/
>
> Signed-off-by: Yang Shi <[email protected]>
> ---
> mm/huge_memory.c | 2 +-
> mm/rmap.c | 2 +-
> 2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 63ed6b25deaa..2122c3e853b9 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2718,7 +2718,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
> }
>
> unmap_page(head);
> - VM_BUG_ON_PAGE(compound_mapcount(head), head);
> + VM_BUG_ON_PAGE(total_mapcount(head), head);
I am not sure about this change. The code below also checks total_mapcount(head)
and returns EBUSY if the count is non-zero. This change makes the code dead.
On the other hand, the change will force all mappings to the page have to be
successfully unmapped all the time. I am not sure if we want to do that.
Maybe it is better to just check total_mapcount() and fail the split.
The same situation happens with the code change below.
>
> /* block interrupt reentry in xa_lock and spinlock */
> local_irq_disable();
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 693a610e181d..2e547378ab5f 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1777,7 +1777,7 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
> else
> rmap_walk(page, &rwc);
>
> - return !page_mapcount(page) ? true : false;
> + return !total_mapcount(page) ? true : false;
> }
In unmap_page(), VM_BUG_ON_PAGE(!unmap_success, page) will force all mappings
to the page have to be all unmapped, which might not be the case we want.
Maybe you will want to remove the VM_BUG_ON_PAGE here, check total_mapcount()
above, and fail the split if not all mappings to the pages are unmapped.
—
Best Regards,
Yan Zi
On Fri, Apr 30, 2021 at 2:30 PM Zi Yan <[email protected]> wrote:
>
> On 30 Apr 2021, at 17:07, Yang Shi wrote:
>
> > When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
> > return false positive for PTE-mapped THP since page_mapcount() is used
> > to check if the THP is unmapped, but it just checks compound mapount and
> > head page's mapcount. If the THP is PTE-mapped and head page is not
> > mapped, it may return false positive.
> >
> > Use total_mapcount() instead of page_mapcount() and do so for the
> > VM_BUG_ON_PAGE in split_huge_page_to_list as well.
> >
> > [1] https://lore.kernel.org/linux-mm/[email protected]/
> >
> > Signed-off-by: Yang Shi <[email protected]>
> > ---
> > mm/huge_memory.c | 2 +-
> > mm/rmap.c | 2 +-
> > 2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 63ed6b25deaa..2122c3e853b9 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2718,7 +2718,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
> > }
> >
> > unmap_page(head);
> > - VM_BUG_ON_PAGE(compound_mapcount(head), head);
> > + VM_BUG_ON_PAGE(total_mapcount(head), head);
>
> I am not sure about this change. The code below also checks total_mapcount(head)
> and returns EBUSY if the count is non-zero. This change makes the code dead.
It is actually dead if CONFIG_DEBUG_VM is enabled and total_mapcount
is not 0 regardless of this change due to the below code, right?
if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
pr_alert("total_mapcount: %u, page_count(): %u\n",
mapcount, count);
if (PageTail(page))
dump_page(head, NULL);
dump_page(page, "total_mapcount(head) > 0");
BUG();
}
> On the other hand, the change will force all mappings to the page have to be
> successfully unmapped all the time. I am not sure if we want to do that.
> Maybe it is better to just check total_mapcount() and fail the split.
> The same situation happens with the code change below.
IIUC, the code did force all mappings to the page to be unmapped in
order to split it.
>
> >
> > /* block interrupt reentry in xa_lock and spinlock */
> > local_irq_disable();
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 693a610e181d..2e547378ab5f 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1777,7 +1777,7 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
> > else
> > rmap_walk(page, &rwc);
> >
> > - return !page_mapcount(page) ? true : false;
> > + return !total_mapcount(page) ? true : false;
> > }
>
> In unmap_page(), VM_BUG_ON_PAGE(!unmap_success, page) will force all mappings
> to the page have to be all unmapped, which might not be the case we want.
AFAICT, I don't see such a case from all the callers of
try_to_unmap(). Imay miss something, but I do have a hard time
thinking of a usecase which can proceed safely with "not fully
unmapped" page.
> Maybe you will want to remove the VM_BUG_ON_PAGE here, check total_mapcount()
> above, and fail the split if not all mappings to the pages are unmapped.
>
>
>
> —
> Best Regards,
> Yan Zi
On 30 Apr 2021, at 17:56, Yang Shi wrote:
> On Fri, Apr 30, 2021 at 2:30 PM Zi Yan <[email protected]> wrote:
>>
>> On 30 Apr 2021, at 17:07, Yang Shi wrote:
>>
>>> When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
>>> return false positive for PTE-mapped THP since page_mapcount() is used
>>> to check if the THP is unmapped, but it just checks compound mapount and
>>> head page's mapcount. If the THP is PTE-mapped and head page is not
>>> mapped, it may return false positive.
>>>
>>> Use total_mapcount() instead of page_mapcount() and do so for the
>>> VM_BUG_ON_PAGE in split_huge_page_to_list as well.
>>>
>>> [1] https://lore.kernel.org/linux-mm/[email protected]/
>>>
>>> Signed-off-by: Yang Shi <[email protected]>
>>> ---
>>> mm/huge_memory.c | 2 +-
>>> mm/rmap.c | 2 +-
>>> 2 files changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 63ed6b25deaa..2122c3e853b9 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -2718,7 +2718,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>>> }
>>>
>>> unmap_page(head);
>>> - VM_BUG_ON_PAGE(compound_mapcount(head), head);
>>> + VM_BUG_ON_PAGE(total_mapcount(head), head);
>>
>> I am not sure about this change. The code below also checks total_mapcount(head)
>> and returns EBUSY if the count is non-zero. This change makes the code dead.
>
> It is actually dead if CONFIG_DEBUG_VM is enabled and total_mapcount
> is not 0 regardless of this change due to the below code, right?
>
> if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
> pr_alert("total_mapcount: %u, page_count(): %u\n",
> mapcount, count);
> if (PageTail(page))
> dump_page(head, NULL);
> dump_page(page, "total_mapcount(head) > 0");
> BUG();
> }
Right. But with this change, mapcount will never be non-zero. The code above
will be useless and can be removed.
>> On the other hand, the change will force all mappings to the page have to be
>> successfully unmapped all the time. I am not sure if we want to do that.
>> Maybe it is better to just check total_mapcount() and fail the split.
>> The same situation happens with the code change below.
>
> IIUC, the code did force all mappings to the page to be unmapped in
> order to split it.
>>
>>>
>>> /* block interrupt reentry in xa_lock and spinlock */
>>> local_irq_disable();
>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>> index 693a610e181d..2e547378ab5f 100644
>>> --- a/mm/rmap.c
>>> +++ b/mm/rmap.c
>>> @@ -1777,7 +1777,7 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
>>> else
>>> rmap_walk(page, &rwc);
>>>
>>> - return !page_mapcount(page) ? true : false;
>>> + return !total_mapcount(page) ? true : false;
>>> }
>>
>> In unmap_page(), VM_BUG_ON_PAGE(!unmap_success, page) will force all mappings
>> to the page have to be all unmapped, which might not be the case we want.
>
> AFAICT, I don't see such a case from all the callers of
> try_to_unmap(). Imay miss something, but I do have a hard time
> thinking of a usecase which can proceed safely with "not fully
> unmapped" page.
This code change is correct, but after the change unmap_page() will fire VM_BUG_ON
when not all mappings are unmapped. Along with the change above, we will have
two identical VM_BUG_ONs happen one after another. We might want to remove one
of them.
Also, this changes the semantics of try_to_unmap. The comment for try_to_unmap
might need to be updated.
—
Best Regards,
Yan Zi
On Fri, Apr 30, 2021 at 3:30 PM Zi Yan <[email protected]> wrote:
>
> On 30 Apr 2021, at 17:56, Yang Shi wrote:
>
> > On Fri, Apr 30, 2021 at 2:30 PM Zi Yan <[email protected]> wrote:
> >>
> >> On 30 Apr 2021, at 17:07, Yang Shi wrote:
> >>
> >>> When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
> >>> return false positive for PTE-mapped THP since page_mapcount() is used
> >>> to check if the THP is unmapped, but it just checks compound mapount and
> >>> head page's mapcount. If the THP is PTE-mapped and head page is not
> >>> mapped, it may return false positive.
> >>>
> >>> Use total_mapcount() instead of page_mapcount() and do so for the
> >>> VM_BUG_ON_PAGE in split_huge_page_to_list as well.
> >>>
> >>> [1] https://lore.kernel.org/linux-mm/[email protected]/
> >>>
> >>> Signed-off-by: Yang Shi <[email protected]>
> >>> ---
> >>> mm/huge_memory.c | 2 +-
> >>> mm/rmap.c | 2 +-
> >>> 2 files changed, 2 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> >>> index 63ed6b25deaa..2122c3e853b9 100644
> >>> --- a/mm/huge_memory.c
> >>> +++ b/mm/huge_memory.c
> >>> @@ -2718,7 +2718,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
> >>> }
> >>>
> >>> unmap_page(head);
> >>> - VM_BUG_ON_PAGE(compound_mapcount(head), head);
> >>> + VM_BUG_ON_PAGE(total_mapcount(head), head);
> >>
> >> I am not sure about this change. The code below also checks total_mapcount(head)
> >> and returns EBUSY if the count is non-zero. This change makes the code dead.
> >
> > It is actually dead if CONFIG_DEBUG_VM is enabled and total_mapcount
> > is not 0 regardless of this change due to the below code, right?
> >
> > if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
> > pr_alert("total_mapcount: %u, page_count(): %u\n",
> > mapcount, count);
> > if (PageTail(page))
> > dump_page(head, NULL);
> > dump_page(page, "total_mapcount(head) > 0");
> > BUG();
> > }
>
> Right. But with this change, mapcount will never be non-zero. The code above
> will be useless and can be removed.
Yes, you are correct.
>
> >> On the other hand, the change will force all mappings to the page have to be
> >> successfully unmapped all the time. I am not sure if we want to do that.
> >> Maybe it is better to just check total_mapcount() and fail the split.
> >> The same situation happens with the code change below.
> >
> > IIUC, the code did force all mappings to the page to be unmapped in
> > order to split it.
> >>
> >>>
> >>> /* block interrupt reentry in xa_lock and spinlock */
> >>> local_irq_disable();
> >>> diff --git a/mm/rmap.c b/mm/rmap.c
> >>> index 693a610e181d..2e547378ab5f 100644
> >>> --- a/mm/rmap.c
> >>> +++ b/mm/rmap.c
> >>> @@ -1777,7 +1777,7 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
> >>> else
> >>> rmap_walk(page, &rwc);
> >>>
> >>> - return !page_mapcount(page) ? true : false;
> >>> + return !total_mapcount(page) ? true : false;
> >>> }
> >>
> >> In unmap_page(), VM_BUG_ON_PAGE(!unmap_success, page) will force all mappings
> >> to the page have to be all unmapped, which might not be the case we want.
> >
> > AFAICT, I don't see such a case from all the callers of
> > try_to_unmap(). Imay miss something, but I do have a hard time
> > thinking of a usecase which can proceed safely with "not fully
> > unmapped" page.
>
> This code change is correct, but after the change unmap_page() will fire VM_BUG_ON
> when not all mappings are unmapped. Along with the change above, we will have
> two identical VM_BUG_ONs happen one after another. We might want to remove one
> of them.
Yes. I'd prefer keep the one after unmap_page() since it seems more
obvious. Any objection?
>
> Also, this changes the semantics of try_to_unmap. The comment for try_to_unmap
> might need to be updated.
What comment do you refer to?
>
>
> —
> Best Regards,
> Yan Zi
On 30 Apr 2021, at 18:55, Yang Shi wrote:
> On Fri, Apr 30, 2021 at 3:30 PM Zi Yan <[email protected]> wrote:
>>
>> On 30 Apr 2021, at 17:56, Yang Shi wrote:
>>
>>> On Fri, Apr 30, 2021 at 2:30 PM Zi Yan <[email protected]> wrote:
>>>>
>>>> On 30 Apr 2021, at 17:07, Yang Shi wrote:
>>>>
>>>>> When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
>>>>> return false positive for PTE-mapped THP since page_mapcount() is used
>>>>> to check if the THP is unmapped, but it just checks compound mapount and
>>>>> head page's mapcount. If the THP is PTE-mapped and head page is not
>>>>> mapped, it may return false positive.
>>>>>
>>>>> Use total_mapcount() instead of page_mapcount() and do so for the
>>>>> VM_BUG_ON_PAGE in split_huge_page_to_list as well.
>>>>>
>>>>> [1] https://lore.kernel.org/linux-mm/[email protected]/
>>>>>
>>>>> Signed-off-by: Yang Shi <[email protected]>
>>>>> ---
>>>>> mm/huge_memory.c | 2 +-
>>>>> mm/rmap.c | 2 +-
>>>>> 2 files changed, 2 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>>> index 63ed6b25deaa..2122c3e853b9 100644
>>>>> --- a/mm/huge_memory.c
>>>>> +++ b/mm/huge_memory.c
>>>>> @@ -2718,7 +2718,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>>>>> }
>>>>>
>>>>> unmap_page(head);
>>>>> - VM_BUG_ON_PAGE(compound_mapcount(head), head);
>>>>> + VM_BUG_ON_PAGE(total_mapcount(head), head);
>>>>
>>>> I am not sure about this change. The code below also checks total_mapcount(head)
>>>> and returns EBUSY if the count is non-zero. This change makes the code dead.
>>>
>>> It is actually dead if CONFIG_DEBUG_VM is enabled and total_mapcount
>>> is not 0 regardless of this change due to the below code, right?
>>>
>>> if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
>>> pr_alert("total_mapcount: %u, page_count(): %u\n",
>>> mapcount, count);
>>> if (PageTail(page))
>>> dump_page(head, NULL);
>>> dump_page(page, "total_mapcount(head) > 0");
>>> BUG();
>>> }
>>
>> Right. But with this change, mapcount will never be non-zero. The code above
>> will be useless and can be removed.
>
> Yes, you are correct.
>
>>
>>>> On the other hand, the change will force all mappings to the page have to be
>>>> successfully unmapped all the time. I am not sure if we want to do that.
>>>> Maybe it is better to just check total_mapcount() and fail the split.
>>>> The same situation happens with the code change below.
>>>
>>> IIUC, the code did force all mappings to the page to be unmapped in
>>> order to split it.
>>>>
>>>>>
>>>>> /* block interrupt reentry in xa_lock and spinlock */
>>>>> local_irq_disable();
>>>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>>>> index 693a610e181d..2e547378ab5f 100644
>>>>> --- a/mm/rmap.c
>>>>> +++ b/mm/rmap.c
>>>>> @@ -1777,7 +1777,7 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
>>>>> else
>>>>> rmap_walk(page, &rwc);
>>>>>
>>>>> - return !page_mapcount(page) ? true : false;
>>>>> + return !total_mapcount(page) ? true : false;
>>>>> }
>>>>
>>>> In unmap_page(), VM_BUG_ON_PAGE(!unmap_success, page) will force all mappings
>>>> to the page have to be all unmapped, which might not be the case we want.
>>>
>>> AFAICT, I don't see such a case from all the callers of
>>> try_to_unmap(). Imay miss something, but I do have a hard time
>>> thinking of a usecase which can proceed safely with "not fully
>>> unmapped" page.
>>
>> This code change is correct, but after the change unmap_page() will fire VM_BUG_ON
>> when not all mappings are unmapped. Along with the change above, we will have
>> two identical VM_BUG_ONs happen one after another. We might want to remove one
>> of them.
>
> Yes. I'd prefer keep the one after unmap_page() since it seems more
> obvious. Any objection?
Sounds good to me.
>
>>
>> Also, this changes the semantics of try_to_unmap. The comment for try_to_unmap
>> might need to be updated.
>
> What comment do you refer to?
/**
* try_to_unmap - try to remove all page table mappings to a page
a page -> a page and the compound page it belongs to
* @page: the page to get unmapped
the page -> the page or the subpage of a compound page
* @flags: action and flags
*
* Tries to remove all the page table entries which are mapping this
* page, used in the pageout path. Caller must hold the page lock.
this page -> this page and the compound page it belongs to
Feel free to change the wording if you find better ones.
—
Best Regards,
Yan Zi