On Fri, May 13, 2022 at 4:17 PM Yang Shi <shy828301@xxxxxxxxx> wrote:
>
> On Fri, May 13, 2022 at 4:05 PM Zach O'Keefe <zokeefe@xxxxxxxxxx> wrote:
> >
> > Thanks for the review, David!
> >
> > On Thu, May 12, 2022 at 1:02 PM David Rientjes <rientjes@xxxxxxxxxx> wrote:
> > >
> > > On Wed, 4 May 2022, Zach O'Keefe wrote:
> > >
> > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > > index c94bc43dff3e..6095fcb3f07c 100644
> > > > --- a/mm/khugepaged.c
> > > > +++ b/mm/khugepaged.c
> > > > @@ -92,6 +92,10 @@ struct collapse_control {
> > > >
> > > > /* Last target selected in khugepaged_find_target_node() */
> > > > int last_target_node;
> > > > +
> > > > + struct page *hpage;
> > > > + int (*alloc_charge_hpage)(struct mm_struct *mm,
> > > > + struct collapse_control *cc);
> > > > };
> > > >
> > > > /**
> > >
> > > Embedding this function pointer into collapse_contol seems like it would
> > > need some pretty strong rationale. Not to say that it should be a
> > > non-starter, but I think the changelog needs to clearly indicate why this
> > > is better/cleaner than embedding the needed info for a single allocation
> > > and charge function to use. If the callbacks would truly be so different
> > > that unifying them would be more complex, I think this makes sense.
> >
> > Mostly, this boils down to khugepaged having different a allocation
> > pattern for NUMA/UMA ; the former scans the pages first to determine
> > the right node, the latter preallocates before scanning. khugepaged
> > has the luxury on UMA systems of just holding onto a hugepage
> > indefinitely for the next collapse target.
> >
> > For MADV_COLLAPSE, we never preallocate, and so its pattern doesn't
> > depend on NUMA or UMA configs. Trying to avoid "if (khugepaged) ...
> > else" casing, defining this as a context-defined operation seemed
> > appropriate.
> >
> > Collapsing both alloc and charging together was mostly a code
> > cleanliness decision resulting from not wanting to embed a ->gfp()
> > hook (gfp flags are used both by allocation and memcg charging).
> > Alternatively, a .gfp member could exist - it would just need to be
> > refreshed periodically in the khugepaged codepath.
> >
> > That all said - let me take another crack at seeing if I can make this
> > work without the need for a function pointer here.
>
> I had a patch that removed UMA allocation, please refer to
> https://lore.kernel.org/linux-mm/20210817202146.3218-1-shy828301@xxxxxxxxx/#t
>
> It was not made upstream due to some requests for further cleanup, but
> unfortunately I haven't got time to look into it yet.
>
> If this page were merged, would that make your life easier?
Hey Yang,
First, sorry for missing that patch in the first place. I actually
have some patches queued up that do a similar cleanup of
khugepaged_prealloc_page() that was mentioned, but decided to not
include them here.
Second, removing the NUMA/UMA story does make this patch easier, I
think (esp since the sched change was dropped for now). This is
something I wanted while writing this series, but without the larger
context referenced in your patch (most users don't build NUMA=n even
on single node systems, and the pcp hugepage lists optimization)
couldn't justify myself.
Best,
Zach
