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

[Mel Gorman]

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@xxxxxxxxx>

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@xxxxxxx>

-- 
Mel Gorman
SUSE Labs