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SubjectRe: [PATCH] /proc/PID/smaps: Add PMD migration entry parsing
Date
Konstantin Khlebnikov <khlebnikov@yandex-team.ru> writes:

> On 01/04/2020 05.31, Huang, Ying wrote:
>> Konstantin Khlebnikov <khlebnikov@yandex-team.ru> writes:
>>
>>> On 31/03/2020 11.56, Huang, Ying wrote:
>>>> From: Huang Ying <ying.huang@intel.com>
>>>>
>>>> Now, when read /proc/PID/smaps, the PMD migration entry in page table is simply
>>>> ignored. To improve the accuracy of /proc/PID/smaps, its parsing and processing
>>>> is added.
>>>>
>>>> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
>>>> Cc: Andrea Arcangeli <aarcange@redhat.com>
>>>> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>>>> Cc: Zi Yan <ziy@nvidia.com>
>>>> Cc: Vlastimil Babka <vbabka@suse.cz>
>>>> Cc: Alexey Dobriyan <adobriyan@gmail.com>
>>>> Cc: Michal Hocko <mhocko@suse.com>
>>>> Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
>>>> Cc: "Jérôme Glisse" <jglisse@redhat.com>
>>>> Cc: Yang Shi <yang.shi@linux.alibaba.com>
>>>> ---
>>>> fs/proc/task_mmu.c | 16 ++++++++++++----
>>>> 1 file changed, 12 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>>>> index 8d382d4ec067..b5b3aef8cb3b 100644
>>>> --- a/fs/proc/task_mmu.c
>>>> +++ b/fs/proc/task_mmu.c
>>>> @@ -548,8 +548,17 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
>>>> bool locked = !!(vma->vm_flags & VM_LOCKED);
>>>> struct page *page;
>>>
>>> struct page *page = NULL;
>>
>> Looks good. Will do this in the next version.
>>
>>>> - /* FOLL_DUMP will return -EFAULT on huge zero page */
>>>> - page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
>>>> + if (pmd_present(*pmd)) {
>>>> + /* FOLL_DUMP will return -EFAULT on huge zero page */
>>>> + page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
>>>> + } else if (unlikely(is_swap_pmd(*pmd))) {
>>>> + swp_entry_t entry = pmd_to_swp_entry(*pmd);
>>>> +
>>>> + VM_BUG_ON(!is_migration_entry(entry));
>>>> + page = migration_entry_to_page(entry);
>>>
>>> if (is_migration_entry(entry))
>>> page = migration_entry_to_page(entry);
>>>
>>> Seems safer and doesn't add much code.
>>
>> With this, we lose an opportunity to capture some bugs during debugging.
>> Right?
>
> You can keep VM_BUG_ON or VM_WARN_ON_ONCE
>
> Off-by-page in statistics isn't a big deal and not a good reason to crash (even debug) kernel.
> But for normal build should use safe behaviour if this isn't hard.

Sounds reasonable! Will revise the code. Thanks!

Best Regards,
Huang, Ying

>>
>> Best Regards,
>> Huang, Ying
>>
>>>> + } else {
>>>> + return;
>>>> + }
>>>> if (IS_ERR_OR_NULL(page))
>>>> return;
>>>> if (PageAnon(page))
>>>> @@ -578,8 +587,7 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>>>> ptl = pmd_trans_huge_lock(pmd, vma);
>>>> if (ptl) {
>>>> - if (pmd_present(*pmd))
>>>> - smaps_pmd_entry(pmd, addr, walk);
>>>> + smaps_pmd_entry(pmd, addr, walk);
>>>> spin_unlock(ptl);
>>>> goto out;
>>>> }
>>>>

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