linux-kernel - Re: [v2 PATCH 3/7] mm: thp: refactor NUMA fault handling

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Message-ID: <20210517152721.GW3672@suse.de>
Date:   Mon, 17 May 2021 16:27:21 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Yang Shi <shy828301@...il.com>
Cc:     kirill.shutemov@...ux.intel.com, ziy@...dia.com, mhocko@...e.com,
        ying.huang@...el.com, hughd@...gle.com,
        gerald.schaefer@...ux.ibm.com, hca@...ux.ibm.com,
        gor@...ux.ibm.com, borntraeger@...ibm.com,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH 3/7] mm: thp: refactor NUMA fault handling

On Tue, Apr 13, 2021 at 02:24:12PM -0700, Yang Shi wrote:
> When the THP NUMA fault support was added THP migration was not supported yet.
> So the ad hoc THP migration was implemented in NUMA fault handling.  Since v4.14
> THP migration has been supported so it doesn't make too much sense to still keep
> another THP migration implementation rather than using the generic migration
> code.
> 
> This patch reworked the NUMA fault handling to use generic migration implementation
> to migrate misplaced page.  There is no functional change.
> 
> After the refactor the flow of NUMA fault handling looks just like its
> PTE counterpart:
>   Acquire ptl
>   Prepare for migration (elevate page refcount)
>   Release ptl
>   Isolate page from lru and elevate page refcount
>   Migrate the misplaced THP
> 
> If migration is failed just restore the old normal PMD.
> 
> In the old code anon_vma lock was needed to serialize THP migration
> against THP split, but since then the THP code has been reworked a lot,
> it seems anon_vma lock is not required anymore to avoid the race.
> 
> The page refcount elevation when holding ptl should prevent from THP
> split.
> 
> Use migrate_misplaced_page() for both base page and THP NUMA hinting
> fault and remove all the dead and duplicate code.
> 
> Signed-off-by: Yang Shi <shy828301@...il.com>

I did not spot any big problems and FWIW, the series overall passed a
series of tests that exercise NUMA balancing migrations so...

Acked-by: Mel Gorman <mgorman@...e.de>

-- 
Mel Gorman
SUSE Labs