Re: Mapcount of subpages

[Zi Yan <ziy@nvidia.com>]

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On 23 Sep 2021, at 19:48, Hugh Dickins wrote:

> On Thu, 23 Sep 2021, Zi Yan wrote:
>> On 23 Sep 2021, at 17:54, Yang Shi wrote:
>>> On Thu, Sep 23, 2021 at 2:10 PM Hugh Dickins <hughd@google.com> wrote:
>>>>
>>>> NR_FILE_MAPPED being used for /proc/meminfo's "Mapped:" and a couple
>>>> of other such stats files, and for a reclaim heuristic in mm/vmscan.c.
>>>>
>>>> Allow ourselves more slack in NR_FILE_MAPPED accounting (either count
>>>> each pte as if it mapped the whole THP, or don't count a THP's ptes
>>>> at all - you opted for the latter in the "Mlocked:" accounting),
>>>> and I suspect subpage _mapcount could be abandoned.
>>>
>>> AFAIK, partial THP unmap may need the _mapcount information of every
>>> subpage otherwise the deferred split can't know what subpages could be
>>> freed.
>
> I believe Yang Shi is right insofar as the decision on whether it's worth
> queuing for deferred split is being done based on those subpage _mapcounts.
> That is a use I had not considered, and I've given no thought to how
> important or not it is.
>
>>
>> Could we just scan page tables of a THP during deferred split process
>> instead? Deferred split is a slow path already, so maybe it can afford
>> the extra work.
>
> But unless I misunderstand, actually carrying out the deferred split
> already unmaps, uses migration entries, and remaps the remaining ptes:
> needing no help from subpage _mapcounts to do those, and free the rest.

You are right. unmap_page() during THP split is scanning the page tables
already.

For deciding whether to queue a THP for deferred split, we probably can
keep PageDoubleMap bit to indicate if any subpage is PTE mapped.

But without subpage _mapcount, detecting extra pins to a THP before split
might be not as easy as with it. This means every THP split will need to
perform unmap_page(), then check the remaining page_count to see if
THP split is possible. That would also introduce extra system-wide overheads
from unmapping pages. Am I missing anything?


--
Best Regards,
Yan, Zi

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