Re: [PATCH 1/1] mm/khugepaged: skip copying lazyfree pages on collapse

[Lance Yang]

I'd like to add one more point.

Users mark pages as lazyfree with MADV_FREE,
expecting these pages to be reclaimed until memory
pressure occurs. Currently, khugepaged treats lazyfree
pages the same as pte_none, which seems reasonable.
IMO, avoiding the copying of these pages to the new huge
page during khugepaged collapse can better keep the
semantics of MADV_FREE. It appears that lazyfree
pages are reclaimed before khugepaged collapses them.

Thanks,
Lance

On Fri, Feb 2, 2024 at 9:46 PM Lance Yang <ioworker0@xxxxxxxxx> wrote:
>
> Here is a part from the man page explaining
> the MADV_FREE semantics:
>
> The kernel can thus free thesepages, but the
> freeing could be delayed until memory pressure
> occurs. For each of the pages that has been
> marked to be freed but has not yet been freed,
> the free operation will be canceled if the caller
> writes into the page. If there is no subsequent
> write, the kernel can free the pages at any time.
>
> IIUC, if there is no subsequent write, lazyfree
> pages will eventually be reclaimed. khugepaged
> treats lazyfree pages the same as pte_none,
> avoiding copying them to the new huge page
> during collapse. It seems that lazyfree pages
> are reclaimed before khugepaged collapses them.
> This aligns with user expectations.
>
> However, IMO, if the content of MADV_FREE pages
> remains valid during collapse, then khugepaged
> treating lazyfree pages the same as pte_none
> might not be suitable.
>
> Thanks,
> Lance
>
> On Fri, Feb 2, 2024 at 8:57 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > On Fri 02-02-24 20:52:48, Lance Yang wrote:
> > > On Fri, Feb 2, 2024 at 8:27 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > > >
> > > > On Fri 02-02-24 19:18:31, Lance Yang wrote:
> > > > > IMO, since it's treated the same as pte_none,
> > > > > perhaps lazyfree pages shouldn't be copied to
> > > > > the new huge page.
> > > >
> > > > Why? The content of MADV_FREE page is valid until it is reclaimed.
> > >
> > > IMO, if MADV_FREE pages are considered valid until
> > > reclaimed, treating them the same as pte_none might
> > > pose a conflict.
> >
> > What kind of conflict?
> > --
> > Michal Hocko
> > SUSE Labs