Re: [RFC 0/3] mm: Discard lazily freed pages when migrating

[Mel Gorman]

On Wed, Mar 04, 2020 at 07:15:20PM +0800, Huang, Ying wrote:
> In which situation the cost to reconstruct MADV_FREE pages can be higher
> than the cost to allocate file cache page and read from disk?  Heavy
> contention on mmap_sem?
> 

MADV_FREE should be anonymous only

if (behavior == MADV_FREE)
                return madvise_free_single_vma(vma, start, end);

.....

static int madvise_free_single_vma(struct vm_area_struct *vma,
                        unsigned long start_addr, unsigned long end_addr)
{
        struct mm_struct *mm = vma->vm_mm;
        struct mmu_notifier_range range;
        struct mmu_gather tlb;

        /* MADV_FREE works for only anon vma at the moment */
        if (!vma_is_anonymous(vma))
                return -EINVAL

So the question is not applicable. For anonymous memory, the cost of
updating a PTE is lower than allocating a page, zeroing it and updating
the PTE.

It has been repeatedly stated now for almost a week that a semantic
change to MADV_FREE should be based on a problem encountered by a real
application that can benefit from the new semantics. I think the only
concrete outcome has been that userspace potentially benefits if the total
number of MADV_FREE pages is reported globally. Even that is marginal as
smaps has the information to tell the difference between high RSS due to
a memory leak and high RSS usage due to MADV_FREE. The /proc/vmstats for
MADV_FREE are of marginal benefit given that they do not tell us much
about the current number of MADV_FREE pages in the system.

-- 
Mel Gorman
SUSE Labs