Re: [PATCH] mm: thp: check total_mapcount instead of page_mapcount

["Zi Yan" <ziy@nvidia.com>]

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On 30 Apr 2021, at 17:56, Yang Shi wrote:

> On Fri, Apr 30, 2021 at 2:30 PM Zi Yan <ziy@nvidia.com> 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/20210412180659.B9E3.409509F4@e16-tech.com/
>>>
>>> Signed-off-by: Yang Shi <shy828301@gmail.com>
>>> ---
>>>  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

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