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

[David Hildenbrand <david@redhat.com>]

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