Re: [RFC PATCH] mm: thp: implement THP reservations for anonymous memory

[Zi Yan]

On 9 Nov 2018, at 8:11, Mel Gorman wrote:

> On Fri, Nov 09, 2018 at 03:13:18PM +0300, Kirill A. Shutemov wrote:
>> On Thu, Nov 08, 2018 at 10:48:58PM -0800, Anthony Yznaga wrote:
>>> The basic idea as outlined by Mel Gorman in [2] is:
>>>
>>> 1) On first fault in a sufficiently sized range, allocate a huge page
>>>    sized and aligned block of base pages.  Map the base page
>>>    corresponding to the fault address and hold the rest of the pages in
>>>    reserve.
>>> 2) On subsequent faults in the range, map the pages from the reservation.
>>> 3) When enough pages have been mapped, promote the mapped pages and
>>>    remaining pages in the reservation to a huge page.
>>> 4) When there is memory pressure, release the unused pages from their
>>>    reservations.
>>
>> I haven't yet read the patch in details, but I'm skeptical about the
>> approach in general for few reasons:
>>
>> - PTE page table retracting to replace it with huge PMD entry requires
>>   down_write(mmap_sem). It makes the approach not practical for many
>>   multi-threaded workloads.
>>
>>   I don't see a way to avoid exclusive lock here. I will be glad to
>>   be proved otherwise.
>>
>
> That problem is somewhat fundamental to the mmap_sem itself and
> conceivably it could be alleviated by range-locking (if that gets
> completed). The other thing to bear in mind is the timing. If the
> promotion is in-place due to reservations, there isn't the allocation
> overhead and the hold times *should* be short.
>

Is it possible to convert all these PTEs to migration entries during
the promotion and replace them with a huge PMD entry afterwards?
AFAIK, migrating pages does not require holding a mmap_sem.
Basically, it will act like migrating 512 base pages to a THP without
actually doing the page copy.

--
Best Regards
Yan Zi

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