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

[Huang\, Ying]

Mel Gorman <mgorman@xxxxxxx> writes:

> 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.

Sorry for confusing.  The original question is NOT about comparing the
reconstruction cost between MADV_FREE anon pages and MADV_FREE file
pages, BUT about comparing the reconstruction cost between MADV_FREE
anon pages and ordinary clean file cache pages.  You can find more
details in conversation between Michal and me.

> 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.

Got it!  Thanks a lot for your patience and sharing.  That's very
helpful.

Best Regards,
Huang, Ying