Re: [PATCH v8 06/10] mm: thp: check pmd migration entry in common path

[Zi Yan]

On 14 Jul 2017, at 5:29, Naoya Horiguchi wrote:

> On Sat, Jul 01, 2017 at 09:40:04AM -0400, Zi Yan wrote:
>> From: Zi Yan <zi.yan@xxxxxxxxxxxxxx>
>>
>> If one of callers of page migration starts to handle thp,
>> memory management code start to see pmd migration entry, so we need
>> to prepare for it before enabling. This patch changes various code
>> point which checks the status of given pmds in order to prevent race
>> between thp migration and the pmd-related works.
>>
>> ChangeLog v1 -> v2:
>> - introduce pmd_related() (I know the naming is not good, but can't
>>   think up no better name. Any suggesntion is welcomed.)
>>
>> Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
>>
>> ChangeLog v2 -> v3:
>> - add is_swap_pmd()
>> - a pmd entry should be pmd pointing to pte pages, is_swap_pmd(),
>>   pmd_trans_huge(), pmd_devmap(), or pmd_none()
>> - pmd_none_or_trans_huge_or_clear_bad() and pmd_trans_unstable() return
>>   true on pmd_migration_entry, so that migration entries are not
>>   treated as pmd page table entries.
>>
>> ChangeLog v4 -> v5:
>> - add explanation in pmd_none_or_trans_huge_or_clear_bad() to state
>>   the equivalence of !pmd_present() and is_pmd_migration_entry()
>> - fix migration entry wait deadlock code (from v1) in follow_page_mask()
>> - remove unnecessary code (from v1) in follow_trans_huge_pmd()
>> - use is_swap_pmd() instead of !pmd_present() for pmd migration entry,
>>   so it will not be confused with pmd_none()
>> - change author information
>>
>> ChangeLog v5 -> v7
>> - use macro to disable the code when thp migration is not enabled
>>
>> ChangeLog v7 -> v8
>> - remove not used code in do_huge_pmd_wp_page()
>> - copy the comment from change_pte_range() on downgrading
>>   write migration entry to read to change_huge_pmd()
>>
>> Signed-off-by: Zi Yan <zi.yan@xxxxxxxxxxxxxx>
>> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>> ---
>>  arch/x86/mm/gup.c             |  7 +++--
>>  fs/proc/task_mmu.c            | 33 ++++++++++++++-------
>>  include/asm-generic/pgtable.h | 17 ++++++++++-
>>  include/linux/huge_mm.h       | 14 +++++++--
>>  mm/gup.c                      | 22 ++++++++++++--
>>  mm/huge_memory.c              | 67 +++++++++++++++++++++++++++++++++++++++----
>>  mm/memcontrol.c               |  5 ++++
>>  mm/memory.c                   | 12 ++++++--
>>  mm/mprotect.c                 |  4 +--
>>  mm/mremap.c                   |  2 +-
>>  10 files changed, 154 insertions(+), 29 deletions(-)
>>
>> diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
>> index 456dfdfd2249..096bbcc801e6 100644
>> --- a/arch/x86/mm/gup.c
>> +++ b/arch/x86/mm/gup.c
>> @@ -9,6 +9,7 @@
>>  #include <linux/vmstat.h>
>>  #include <linux/highmem.h>
>>  #include <linux/swap.h>
>> +#include <linux/swapops.h>
>>  #include <linux/memremap.h>
>>
>>  #include <asm/mmu_context.h>
>> @@ -243,9 +244,11 @@ static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
>>  		pmd_t pmd = *pmdp;
>>
>>  		next = pmd_addr_end(addr, end);
>> -		if (pmd_none(pmd))
>> +		if (!pmd_present(pmd)) {
>> +			VM_BUG_ON(is_swap_pmd(pmd) && IS_ENABLED(CONFIG_MIGRATION) &&
>> +					  !is_pmd_migration_entry(pmd));
>
> This VM_BUG_ON() triggers when gup is called on hugetlb hwpoison entry.
> I think that in such case kernel falls into the gup slow path, and
> a page fault in follow_hugetlb_page() can properly report the error to
> affected processes, so no need to alarm with BUG_ON.
>
> Could you make this VM_BUG_ON more specific, or just remove it?

I will remove it, since adding code to detect hugetlb hwpoison entry
to existing VM_BUG_ON() will be quite messy.

Thanks for pointing this out.

--
Best Regards
Yan Zi

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