On 10.11.21 08:03, Peter Xu wrote:
> Hi, Mina,
>
> Sorry to comment late.
>
> On Sun, Nov 07, 2021 at 03:57:54PM -0800, Mina Almasry wrote:
>> diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
>> index fdc19fbc10839..8a0f0064ff336 100644
>> --- a/Documentation/admin-guide/mm/pagemap.rst
>> +++ b/Documentation/admin-guide/mm/pagemap.rst
>> @@ -23,7 +23,8 @@ There are four components to pagemap:
>> * Bit 56 page exclusively mapped (since 4.2)
>> * Bit 57 pte is uffd-wp write-protected (since 5.13) (see
>> :ref:`Documentation/admin-guide/mm/userfaultfd.rst <userfaultfd>`)
>> - * Bits 57-60 zero
>> + * Bit 58 page is a huge (PMD size) THP mapping
>> + * Bits 59-60 zero
>> * Bit 61 page is file-page or shared-anon (since 3.5)
>> * Bit 62 page swapped
>> * Bit 63 page present
>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>> index ad667dbc96f5c..6f1403f83b310 100644
>> --- a/fs/proc/task_mmu.c
>> +++ b/fs/proc/task_mmu.c
>> @@ -1302,6 +1302,7 @@ struct pagemapread {
>> #define PM_SOFT_DIRTY BIT_ULL(55)
>> #define PM_MMAP_EXCLUSIVE BIT_ULL(56)
>> #define PM_UFFD_WP BIT_ULL(57)
>> +#define PM_HUGE_THP_MAPPING BIT_ULL(58)
>
> The ending "_MAPPING" seems redundant to me, how about just call it "PM_THP" or
> "PM_HUGE" (as THP also means HUGE already)?
>
> IMHO the core problem is about permission controls, and it seems to me we're
> actually trying to workaround it by duplicating some information we have.. so
> it's kind of a pity. Totally not against this patch, but imho it'll be nicer
> if it's the permission part that to be enhanced, rather than a new but slightly
> duplicated interface.
It's not a permission problem AFAIKS: even with permissions "changed",
any attempt to use /proc/kpageflags is just racy. Let's not go down that
path, it's really the wrong mechanism to export to random userspace.
We do have an interface to access this information from userspace
already: /proc/self/smaps IIRC. Mina commented that they are seeing
performance issues with that approach.
It would be valuable to add these details to the patch description,
including a performance difference when using both interfaces we have
available. As the patch description stands, there is no explanation
"why" we want this change.
--
Thanks,
David / dhildenb
