Re: [Qemu-devel] [PATCH v8 51/54] Postcopy: Mark nohugepage before discard

[Juan Quintela]

"Dr. David Alan Gilbert (git)" <address@hidden> wrote:
> From: "Dr. David Alan Gilbert" <address@hidden>
>
> Prior to servicing userfault requests we must ensure we've not got
> huge pages in the area that might include non-transferred memory,
> since a hugepage could incorrectly mark the whole huge page as present.
>
> We mark the area as non-huge page (nhp) just before we perform
> discards; the discard code now tells us to discard any areas
> that haven't been sent (as well as any that are redirtied);
> any already formed transparent-huge-pages get fragmented
> by this discard process if they cotnain any discards.
>
> Transparent huge pages that have been entirely transferred
> and don't contain any discards are not broken by this mechanism;
> they stay as huge pages.
>
> By starting postcopy after a full precopy pass, many of the pages
> then stay as huge pages; this is important for maintaining performance
> after the end of the migration.
>
> Signed-off-by: Dr. David Alan Gilbert <address@hidden>

Reviewed-by: Juan Quintela <address@hidden>