Re: [PATCH v4 3/9] mm: pagewalk: Don't split transhuge pmds when a pmd_entry is present

[Thomas HellstrÃm (VMware)]

Hi, Kirill.

Thanks for reviewing.

On 10/9/19 5:27 PM, Kirill A. Shutemov wrote:
> On Tue, Oct 08, 2019 at 11:15:02AM +0200, Thomas HellstrÃm (VMware) wrote:
> > From: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
> >
> > The pagewalk code was unconditionally splitting transhuge pmds when a
> > pte_entry was present. However ideally we'd want to handle transhuge pmds
> > in the pmd_entry function and ptes in pte_entry function. So don't split
> > huge pmds when there is a pmd_entry function present, but let the callback
> > take care of it if necessary.
> Do we have any current user that expect split_huge_pmd() in this scenario.

No. All current users either have pmd_entry (no splitting) or pte_entry 
(unconditional splitting)

> > In order to make sure a virtual address range is handled by one and only
> > one callback, and since pmd entries may be unstable, we introduce a
> > pmd_entry return code that tells the walk code to continue processing this
> > pmd entry rather than to move on. Since caller-defined positive return
> > codes (up to 2) are used by current callers, use a high value that allows a
> > large range of positive caller-defined return codes for future users.
> >
> > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > Cc: Will Deacon <will.deacon@xxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Rik van Riel <riel@xxxxxxxxxxx>
> > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Huang Ying <ying.huang@xxxxxxxxx>
> > Cc: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> > Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
> > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
> > ---
> >   include/linux/pagewalk.h |  8 ++++++++
> >   mm/pagewalk.c            | 28 +++++++++++++++++++++-------
> >   2 files changed, 29 insertions(+), 7 deletions(-)
> >
> > diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
> > index bddd9759bab9..c4a013eb445d 100644
> > --- a/include/linux/pagewalk.h
> > +++ b/include/linux/pagewalk.h
> > @@ -4,6 +4,11 @@
> >   
> >   #include <linux/mm.h>
> >   
> > +/* Highest positive pmd_entry caller-specific return value */
> > +#define PAGE_WALK_CALLER_MAX     (INT_MAX / 2)
> > +/* The handler did not handle the entry. Fall back to the next level */
> > +#define PAGE_WALK_FALLBACK       (PAGE_WALK_CALLER_MAX + 1)
> > +
> That's hacky.
>
> Maybe just use an error code for this? -EAGAIN?

I agree this is hacky. But IMO it's a reasonably safe option. My 
thinking was that in the long run we'd move the positive return codes to 
the mm_walk private and introduce a PAGE_WALK_TERMINATE code as well.

Perhaps a completely clean and safe way would be to add an "int 
walk_control" in the struct mm_walk?

I'm pretty sure using an error code will come back and bite us at some 
point, if someone just blindly forwards error messages. But if you 
insist, I'll use -EAGAIN.

Please let me know what you think.

Thanks,

Thomas