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

[Lance Yang <ioworker0@gmail.com>]

How about blocking khugepaged from
collapsing lazyfree pages? This way,
is it not better to keep the semantics
of MADV_FREE?

What do you think?

Thanks,
Lance

On Fri, Feb 2, 2024 at 10:42 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Fri 02-02-24 21:46:45, Lance Yang 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.
>
> If there is no memory pressure then this might not
> ever happen. User cannot make any assumption about
> their content once madvise call has been done. The
> content has to be considered lost. Sure the userspace
> might have means to tell those pages from zero pages
> and recheck after the write but that is about it.
>
> > 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.
>
> Why?
>
> Unless I am missing something (which is possible of
> course) I do not really see why dropping the content
> of those pages and replacing them with a THP is any
> difference from reclaiming those pages and then faulting
> in a non-THP zero page.
>
> Now, if khugepaged reused the original content of MADV_FREE
> pages that would be a slightly different story. I can
> see why users would expect zero pages to back madvised
> area.
> --
> Michal Hocko
> SUSE Labs