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

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Date:   Fri, 09 Nov 2018 10:34:07 -0500
From:   "Zi Yan" <zi.yan@...rutgers.edu>
To:     "Mel Gorman" <mgorman@...hsingularity.net>,
        "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     "Anthony Yznaga" <anthony.yznaga@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, aarcange@...hat.com,
        aneesh.kumar@...ux.ibm.com, akpm@...ux-foundation.org,
        jglisse@...hat.com, khandual@...ux.vnet.ibm.com,
        kirill.shutemov@...ux.intel.com, mhocko@...nel.org,
        minchan@...nel.org, peterz@...radead.org, rientjes@...gle.com,
        vbabka@...e.cz, willy@...radead.org, ying.huang@...el.com,
        nitingupta910@...il.com
Subject: Re: [RFC PATCH] mm: thp: implement THP reservations for anonymous
 memory

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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