linux-kernel - Re: [PATCH v1 2/2] mm/migrate: move NUMA hinting fault folio isolation + checks under PTL

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Message-ID: <B726B923-E023-4E56-BED1-BFA5CEF644A1@nvidia.com>
Date: Fri, 21 Jun 2024 09:44:38 -0400
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v1 2/2] mm/migrate: move NUMA hinting fault folio
 isolation + checks under PTL

On 20 Jun 2024, at 17:29, David Hildenbrand wrote:

> Currently we always take a folio reference even if migration will not
> even be tried or isolation failed, requiring us to grab+drop an additional
> reference.
>
> Further, we end up calling folio_likely_mapped_shared() while the folio
> might have already been unmapped, because after we dropped the PTL, that
> can easily happen. We want to stop touching mapcounts and friends from
> such context, and only call folio_likely_mapped_shared() while the folio
> is still mapped: mapcount information is pretty much stale and unreliable
> otherwise.
>
> So let's move checks into numamigrate_isolate_folio(), rename that
> function to migrate_misplaced_folio_prepare(), and call that function
> from callsites where we call migrate_misplaced_folio(), but still with
> the PTL held.
>
> We can now stop taking temporary folio references, and really only take
> a reference if folio isolation succeeded. Doing the
> folio_likely_mapped_shared() + golio isolation under PT lock is now similar
> to how we handle MADV_PAGEOUT.
>
> While at it, combine the folio_is_file_lru() checks.
>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  include/linux/migrate.h |  7 ++++
>  mm/huge_memory.c        |  8 ++--
>  mm/memory.c             |  9 +++--
>  mm/migrate.c            | 81 +++++++++++++++++++----------------------
>  4 files changed, 55 insertions(+), 50 deletions(-)

LGTM. Reviewed-by: Zi Yan <ziy@...dia.com>

One nit below:

<snip>

> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index fc27dabcd8e3..4b2817bb2c7d 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1688,11 +1688,13 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf)
>  	if (node_is_toptier(nid))
>  		last_cpupid = folio_last_cpupid(folio);
>  	target_nid = numa_migrate_prep(folio, vmf, haddr, nid, &flags);
> -	if (target_nid == NUMA_NO_NODE) {
> -		folio_put(folio);
> +	if (target_nid == NUMA_NO_NODE)
> +		goto out_map;
> +	if (migrate_misplaced_folio_prepare(folio, vma, target_nid)) {
> +		flags |= TNF_MIGRATE_FAIL;
>  		goto out_map;
>  	}
> -
> +	/* The folio is isolated and isolation code holds a folio reference. */
>  	spin_unlock(vmf->ptl);
>  	writable = false;
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 118660de5bcc..4fd1ecfced4d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c

<snip>

> @@ -5345,10 +5343,13 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
>  	else
>  		last_cpupid = folio_last_cpupid(folio);
>  	target_nid = numa_migrate_prep(folio, vmf, vmf->address, nid, &flags);
> -	if (target_nid == NUMA_NO_NODE) {
> -		folio_put(folio);
> +	if (target_nid == NUMA_NO_NODE)
> +		goto out_map;
> +	if (migrate_misplaced_folio_prepare(folio, vma, target_nid)) {
> +		flags |= TNF_MIGRATE_FAIL;
>  		goto out_map;
>  	}

These two locations are repeated code, maybe just merge the ifs into
numa_migrate_prep(). Feel free to ignore if you are not going to send
another version. :)

--
Best Regards,
Yan, Zi

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