Re: [PATCH v21 1/4] mm: add VM_DROPPABLE for designating always lazily freeable mappings

[David Hildenbrand]

On 08.07.24 22:21, David Hildenbrand wrote:
> On 08.07.24 16:40, Jason A. Donenfeld wrote:
> > Hi David, Linus,
> >
> > Below is what I understand the suggestions about the UX to be. The full
> > commit is in https://git.zx2c4.com/linux-rng/log/ but here's the part
> > we've been discussing. I've held off on David's suggestion changing
> > "DROPPABLE" to "VOLATILE" to give Linus some time to wake up on the west
> > coast and voice his preference for "DROPPABLE". But the rest is in
> > place.
> >
> > Jason
> >
> > diff --git a/include/uapi/linux/mman.h b/include/uapi/linux/mman.h
> > index a246e11988d5..e89d00528f2f 100644
> > --- a/include/uapi/linux/mman.h
> > +++ b/include/uapi/linux/mman.h
> > @@ -17,6 +17,7 @@
> >    #define MAP_SHARED	0x01		/* Share changes */
> >    #define MAP_PRIVATE	0x02		/* Changes are private */
> >    #define MAP_SHARED_VALIDATE 0x03	/* share + validate extension flags */
> > +#define MAP_DROPPABLE	0x08		/* Zero memory under memory pressure. */
> >    
> >    /*
> >     * Huge page size encoding when MAP_HUGETLB is specified, and a huge page
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index a77893462b92..cba5bc652fc4 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -1068,13 +1068,16 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
> >    		new_flags |= VM_WIPEONFORK;
> >    		break;
> >    	case MADV_KEEPONFORK:
> > +		if (vma->vm_flags & VM_DROPPABLE)
> > +			return -EINVAL;
> >    		new_flags &= ~VM_WIPEONFORK;
> >    		break;
> >    	case MADV_DONTDUMP:
> >    		new_flags |= VM_DONTDUMP;
> >    		break;
> >    	case MADV_DODUMP:
> > -		if (!is_vm_hugetlb_page(vma) && new_flags & VM_SPECIAL)
> > +		if ((!is_vm_hugetlb_page(vma) && new_flags & VM_SPECIAL) ||
> > +		    (vma->vm_flags & VM_DROPPABLE))
> >    			return -EINVAL;
> >    		new_flags &= ~VM_DONTDUMP;
> >    		break;
> > diff --git a/mm/mlock.c b/mm/mlock.c
> > index 30b51cdea89d..b87b3d8cc9cc 100644
> > --- a/mm/mlock.c
> > +++ b/mm/mlock.c
> > @@ -485,7 +485,7 @@ static int mlock_fixup(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >    
> >    	if (newflags == oldflags || (oldflags & VM_SPECIAL) ||
> >    	    is_vm_hugetlb_page(vma) || vma == get_gate_vma(current->mm) ||
> > -	    vma_is_dax(vma) || vma_is_secretmem(vma))
> > +	    vma_is_dax(vma) || vma_is_secretmem(vma) || (oldflags & VM_DROPPABLE))
> >    		/* don't set VM_LOCKED or VM_LOCKONFAULT and don't count */
> >    		goto out;
> >    
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 83b4682ec85c..b3d38179dd42 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -1369,6 +1369,34 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
> >    			pgoff = 0;
> >    			vm_flags |= VM_SHARED | VM_MAYSHARE;
> >    			break;
> > +		case MAP_DROPPABLE:
> > +			/*
> > +			 * A locked or stack area makes no sense to be droppable.
> > +			 *
> > +			 * Also, since droppable pages can just go away at any time
> > +			 * it makes no sense to copy them on fork or dump them.
> > +			 *
> > +			 * And don't attempt to combine with hugetlb for now.
> > +			 */
> > +			if (flags & (MAP_LOCKED | MAP_HUGETLB))
> > +			        return -EINVAL;
> > +			if (vm_flags & (VM_GROWSDOWN | VM_GROWSUP))
> > +			        return -EINVAL;
> > +
> > +			vm_flags |= VM_DROPPABLE;
> > +
> > +			/*
> > +			 * If the pages can be dropped, then it doesn't make
> > +			 * sense to reserve them.
> > +			 */
> > +			vm_flags |= VM_NORESERVE;
>
> That is certainly interesting. Nothing that we might not be able to

"Nothing" -> "I'll note that" :)

--
Cheers,

David / dhildenb