Re: [PATCH 2/2] mm,swap: skip swap readahead if page was obtained instantaneously

[Rik van Riel <riel@surriel.com>]

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On Tue, 2020-09-22 at 11:13 +0800, huang ying wrote:
> On Tue, Sep 22, 2020 at 10:02 AM Rik van Riel <riel@surriel.com>
> wrote:
> > Check whether a swap page was obtained instantaneously, for example
> > because it is in zswap, or on a very fast IO device which uses busy
> > waiting, and we did not wait on IO to swap in this page.
> > If no IO was needed to get the swap page we want, kicking off
> > readahead
> > on surrounding swap pages is likely to be counterproductive,
> > because the
> > extra loads will cause additional latency, use up extra memory, and
> > chances
> > are the surrounding pages in swap are just as fast to load as this
> > one,
> > making readahead pointless.
> > 
> > Signed-off-by: Rik van Riel <riel@surriel.com>
> > ---
> >  mm/swap_state.c | 14 +++++++++++---
> >  1 file changed, 11 insertions(+), 3 deletions(-)
> > 
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index aacb9ba53f63..6919f9d5fe88 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -637,6 +637,7 @@ static struct page
> > *swap_cluster_read_one(swp_entry_t entry,
> >  struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t
> > gfp_mask,
> >                                 struct vm_fault *vmf)
> 
> Why not do this for swap_vma_readahead()
> too?  swap_cluster_read_one()
> can be used in swap_vma_readahead() too.

Good point, I should do the same thing for swap_vma_readahead()
as well. Let me do that and send in a version 2 of the series.

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