Hello Hannes,
On Tue, Mar 18, 2014 at 02:26:21PM -0400, Johannes Weiner wrote:
> On Fri, Mar 14, 2014 at 03:37:47PM +0900, Minchan Kim wrote:
> > Linux doesn't have an ability to free pages lazy while other OS
> > already have been supported that named by madvise(MADV_FREE).
> >
> > The gain is clear that kernel can evict freed pages rather than
> > swapping out or OOM if memory pressure happens.
> >
> > Without memory pressure, freed pages would be reused by userspace
> > without another additional overhead(ex, page fault + + page allocation
> > + page zeroing).
> >
> > Firstly, heavy users would be general allocators(ex, jemalloc,
> > I hope ptmalloc support it) and jemalloc already have supported
> > the feature for other OS(ex, FreeBSD)
> >
> > At the moment, this patch would break build other ARCHs which have
> > own TLB flush scheme other than that x86 but if there is no objection
> > in this direction, I will add patches for handling other ARCHs
> > in next iteration.
> >
> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
>
> > @@ -284,8 +286,17 @@ static long madvise_dontneed(struct vm_area_struct *vma,
> > .last_index = ULONG_MAX,
> > };
> > zap_page_range(vma, start, end - start, &details);
> > + } else if (behavior == MADV_FREE) {
> > + struct zap_details details = {
> > + .lazy_free = 1,
> > + };
> > +
> > + if (vma->vm_file)
> > + return -EINVAL;
> > + zap_page_range(vma, start, end - start, &details);
>
> Wouldn't a custom page table walker to clear dirty bits and move pages
> be better? It's awkward to hook this into the freeing code and then
> special case the pages and not actually free them.
NP.
>
> > @@ -817,6 +817,25 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> >
> > sc->nr_scanned++;
> >
> > + if (PageLazyFree(page)) {
> > + switch (try_to_unmap(page, ttu_flags)) {
>
> I don't get why we need a page flag for this. page_check_references()
> could use the rmap walk to also check if any pte/pmd is dirty. If so,
> you have to swap the page. If all are clean, it can be discarded.
Ugh, you're right. I guess it could work.
I will look into that in next iteration.
Thanks!
>
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Kind regards,
Minchan Kim
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