[PATCH] mm/page_isolation: clarify FIXME around shrink_slab() in memory hotplug

[PATCH] mm/page_isolation: clarify FIXME around shrink_slab() in memory hotplug

[Manish Kumar]

The existing FIXME comment notes that memory hotplug doesn't invoke
shrink_slab() directly. This patch adds context explaining that this is
an intentional design choice to avoid recursion or deadlocks in the
memory reclaim path, as slab shrinking is handled by vmscan.

Signed-off-by: Manish Kumar <manish1588@gmail.com>
---
 mm/page_isolation.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index b2fc5266e3d2..2ca20c3f0a97 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -176,10 +176,16 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
 
 	/*
 	 * FIXME: Now, memory hotplug doesn't call shrink_slab() by itself.
-	 * We just check MOVABLE pages.
+	 *
+	 * This is an intentional limitation: invoking shrink_slab() from a
+	 * hotplug path can cause reclaim recursion or deadlock if the normal
+	 * memory reclaim (vmscan) path is already active. Slab shrinking is
+	 * handled by the vmscan reclaim code under normal operation, so hotplug
+	 * avoids direct calls into shrink_slab() to prevent reentrancy issues.
+	 *
+	 * We therefore only check MOVABLE pages here.
 	 *
 	 * Pass the intersection of [start_pfn, end_pfn) and the page's pageblock
-	 * to avoid redundant checks.
 	 */
 	check_unmovable_start = max(page_to_pfn(page), start_pfn);
 	check_unmovable_end = min(pageblock_end_pfn(page_to_pfn(page)),
-- 
2.43.0

Re: [PATCH] mm/page_isolation: clarify FIXME around shrink_slab() in memory hotplug

[David Hildenbrand]

On 15.10.25 19:50, Manish Kumar wrote:
> The existing FIXME comment notes that memory hotplug doesn't invoke
> shrink_slab() directly. This patch adds context explaining that this is
> an intentional design choice to avoid recursion or deadlocks in the
> memory reclaim path, as slab shrinking is handled by vmscan.
> 
> Signed-off-by: Manish Kumar <manish1588@gmail.com>
> ---
>   mm/page_isolation.c | 10 ++++++++--
>   1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index b2fc5266e3d2..2ca20c3f0a97 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -176,10 +176,16 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
>   
>   	/*
>   	 * FIXME: Now, memory hotplug doesn't call shrink_slab() by itself.
> -	 * We just check MOVABLE pages.
> +	 *
> +	 * This is an intentional limitation: invoking shrink_slab() from a
> +	 * hotplug path can cause reclaim recursion or deadlock if the normal
> +	 * memory reclaim (vmscan) path is already active. Slab shrinking is
> +	 * handled by the vmscan reclaim code under normal operation, so hotplug
> +	 * avoids direct calls into shrink_slab() to prevent reentrancy issues.

This is the first time I hear about this reentrance issue.

How did you come up with this explanation?

-- 
Cheers

David / dhildenb

Re: [PATCH] mm/page_isolation: clarify FIXME around shrink_slab() in memory hotplug

[Andrew Morton]

On Mon, 20 Oct 2025 21:26:42 +0200 David Hildenbrand <david@redhat.com> wrote:

> On 15.10.25 19:50, Manish Kumar wrote:
> > The existing FIXME comment notes that memory hotplug doesn't invoke
> > shrink_slab() directly. This patch adds context explaining that this is
> > an intentional design choice to avoid recursion or deadlocks in the
> > memory reclaim path, as slab shrinking is handled by vmscan.
> > 
> > Signed-off-by: Manish Kumar <manish1588@gmail.com>
> > ---
> >   mm/page_isolation.c | 10 ++++++++--
> >   1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> > index b2fc5266e3d2..2ca20c3f0a97 100644
> > --- a/mm/page_isolation.c
> > +++ b/mm/page_isolation.c
> > @@ -176,10 +176,16 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
> >   
> >   	/*
> >   	 * FIXME: Now, memory hotplug doesn't call shrink_slab() by itself.
> > -	 * We just check MOVABLE pages.
> > +	 *
> > +	 * This is an intentional limitation: invoking shrink_slab() from a
> > +	 * hotplug path can cause reclaim recursion or deadlock if the normal
> > +	 * memory reclaim (vmscan) path is already active. Slab shrinking is
> > +	 * handled by the vmscan reclaim code under normal operation, so hotplug
> > +	 * avoids direct calls into shrink_slab() to prevent reentrancy issues.
> 
> This is the first time I hear about this reentrance issue.
> 
> How did you come up with this explanation?
> 

-ETIMEDOUT, I'll drop this patch.