Re: [PATCH] mm: migrate: restore the nmask after successfully allocating on the target node

[Simon Wang (王传国)]

> On Wed, Mar 26, 2025 at 11:12:18AM +0800, wangchuanguo wrote:
> > If memory is successfully allocated on the target node and the
> > function directly returns without value restore for nmask, non-first
> > migration operations in migrate_pages() by again label may ignore the
> > nmask settings, thereby allowing new memory allocations for migration
> > on any node.
> 
> I have no opinion on whether this is the right thing to do or not, but if it is
>

I don't think so. When memory allocation fails on the target node, there is already a recovery operation for the nmask value below. Therefore, the nmask value should only be restored when memory allocation is successfully completed on the target node.

> > +++ b/mm/vmscan.c
> > @@ -1026,8 +1026,10 @@ struct folio *alloc_migrate_folio(struct folio *src,
> unsigned long private)
> >  	mtc->nmask = NULL;
> >  	mtc->gfp_mask |= __GFP_THISNODE;
> >  	dst = alloc_migration_target(src, (unsigned long)mtc);
> > -	if (dst)
> > +	if (dst) {
> > +		mtc->nmask = allowed_mask;
> >  		return dst;
> > +	}
> >
> >  	mtc->gfp_mask &= ~__GFP_THISNODE;
> >  	mtc->nmask = allowed_mask;
> 
> this should be:
> 
> 	dst = alloc_migration_target(src, (unsigned long)mtc);
> +	mtc->nmask = allowed_mask;
> 	if (dst)
> 		return dst;
> 
> 	mtc->gfp_mask &= ~__GFP_THISNODE;
> -	mtc->nmask = allowed_mask;