Re: [PATCH 6/8] mm/vmalloc: Defer freeing partly initialized vm_struct

[Uladzislau Rezki <urezki@gmail.com>]

On Mon, Aug 18, 2025 at 12:21:15PM +0800, Baoquan He wrote:
> On 08/07/25 at 09:58am, Uladzislau Rezki (Sony) wrote:
> > __vmalloc_area_node() may call free_vmap_area() or vfree() on
> > error paths, both of which can sleep. This becomes problematic
> > if the function is invoked from an atomic context, such as when
> > GFP_ATOMIC or GFP_NOWAIT is passed via gfp_mask.
> > 
> > To fix this, unify error paths and defer the cleanup of partly
> > initialized vm_struct objects to a workqueue. This ensures that
> > freeing happens in a process context and avoids invalid sleeps
> > in atomic regions.
> > 
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > ---
> >  include/linux/vmalloc.h |  6 +++++-
> >  mm/vmalloc.c            | 34 +++++++++++++++++++++++++++++++---
> >  2 files changed, 36 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> > index fdc9aeb74a44..b1425fae8cbf 100644
> > --- a/include/linux/vmalloc.h
> > +++ b/include/linux/vmalloc.h
> > @@ -50,7 +50,11 @@ struct iov_iter;		/* in uio.h */
> >  #endif
> >  
> >  struct vm_struct {
> > -	struct vm_struct	*next;
> > +	union {
> > +		struct vm_struct *next;	  /* Early registration of vm_areas. */
> > +		struct llist_node llnode; /* Asynchronous freeing on error paths. */
> > +	};
> > +
> >  	void			*addr;
> >  	unsigned long		size;
> >  	unsigned long		flags;
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 7f48a54ec108..2424f80d524a 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -3680,6 +3680,35 @@ vm_area_alloc_pages(gfp_t gfp, int nid,
> >  	return nr_allocated;
> >  }
> >  
> > +static LLIST_HEAD(pending_vm_area_cleanup);
> > +static void cleanup_vm_area_work(struct work_struct *work)
> > +{
> > +	struct vm_struct *area, *tmp;
> > +	struct llist_node *head;
> > +
> > +	head = llist_del_all(&pending_vm_area_cleanup);
> > +	if (!head)
> > +		return;
> > +
> > +	llist_for_each_entry_safe(area, tmp, head, llnode) {
> > +		if (!area->pages)
> > +			free_vm_area(area);
> > +		else
> > +			vfree(area->addr);
> > +	}
> > +}
> > +
> > +/*
> > + * Helper for __vmalloc_area_node() to defer cleanup
> > + * of partially initialized vm_struct in error paths.
> > + */
> > +static DECLARE_WORK(cleanup_vm_area, cleanup_vm_area_work);
> > +static void defer_vm_area_cleanup(struct vm_struct *area)
> > +{
> > +	if (llist_add(&area->llnode, &pending_vm_area_cleanup))
> > +		schedule_work(&cleanup_vm_area);
> > +}
> 
> Wondering why here we need call schudule_work() when
> pending_vm_area_cleanup was empty before adding new entry. Shouldn't
> it be as below to schedule the job? Not sure if I miss anything.
> 
> 	if (!llist_add(&area->llnode, &pending_vm_area_cleanup))
> 		schedule_work(&cleanup_vm_area);
> 
> =====
> /**
>  * llist_add - add a new entry
>  * @new:        new entry to be added
>  * @head:       the head for your lock-less list
>  *
>  * Returns true if the list was empty prior to adding this entry.
>  */
> static inline bool llist_add(struct llist_node *new, struct llist_head *head)
> {
>         return llist_add_batch(new, new, head);
> }
> =====
> 
But then you will not schedule. If the list is empty, we add one element
llist_add() returns 1, but your condition expects 0.

How it works:

If someone keeps adding to the llist and it is not empty we should not
trigger a new work, because a current work is in flight(it will cover new comers),
i.e. it has been scheduled but it has not yet completed llist_del_all() on
the head.

Once it is done, a new comer will trigger a work again only if it sees NULL,
i.e. when the list is empty.

--
Uladzislau Rezki