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SubjectRe: [PATCH -mm -v10 1/3] mm, THP, swap: Delay splitting THP during swap out
On Wed, May 10, 2017 at 09:56:54AM -0400, Johannes Weiner wrote:
> Hi Michan,
>
> On Tue, May 02, 2017 at 08:53:32AM +0900, Minchan Kim wrote:
> > @@ -1144,7 +1144,7 @@ void swap_free(swp_entry_t entry)
> > /*
> > * Called after dropping swapcache to decrease refcnt to swap entries.
> > */
> > -void swapcache_free(swp_entry_t entry)
> > +void __swapcache_free(swp_entry_t entry)
> > {
> > struct swap_info_struct *p;
> >
> > @@ -1156,7 +1156,7 @@ void swapcache_free(swp_entry_t entry)
> > }
> >
> > #ifdef CONFIG_THP_SWAP
> > -void swapcache_free_cluster(swp_entry_t entry)
> > +void __swapcache_free_cluster(swp_entry_t entry)
> > {
> > unsigned long offset = swp_offset(entry);
> > unsigned long idx = offset / SWAPFILE_CLUSTER;
> > @@ -1182,6 +1182,14 @@ void swapcache_free_cluster(swp_entry_t entry)
> > }
> > #endif /* CONFIG_THP_SWAP */
> >
> > +void swapcache_free(struct page *page, swp_entry_t entry)
> > +{
> > + if (!PageTransHuge(page))
> > + __swapcache_free(entry);
> > + else
> > + __swapcache_free_cluster(entry);
> > +}
>
> I don't think this is cleaner :/
>
> On your second patch:
>
> > @@ -1125,8 +1125,28 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> > !PageSwapCache(page)) {
> > if (!(sc->gfp_mask & __GFP_IO))
> > goto keep_locked;
> > - if (!add_to_swap(page, page_list))
> > +swap_retry:
> > + /*
> > + * Retry after split if we fail to allocate
> > + * swap space of a THP.
> > + */
> > + if (!add_to_swap(page)) {
> > + if (!PageTransHuge(page) ||
> > + split_huge_page_to_list(page, page_list))
> > + goto activate_locked;
> > + goto swap_retry;
> > + }
>
> This is definitely better.

Thanks.

>
> However, I think it'd be cleaner without the label here:
>
> if (!add_to_swap(page)) {
> if (!PageTransHuge(page))
> goto activate_locked;
> /* Split THP and swap individual base pages */
> if (split_huge_page_to_list(page, page_list))
> goto activate_locked;
> if (!add_to_swap(page))
> goto activate_locked;

Yes.

> }
>
> > + /*
> > + * Got swap space successfully. But unfortunately,
> > + * we don't support a THP page writeout so split it.
> > + */
> > + if (PageTransHuge(page) &&
> > + split_huge_page_to_list(page, page_list)) {
> > + delete_from_swap_cache(page);
> > goto activate_locked;
> > + }
>
> Pulling this out of add_to_swap() is an improvement for sure. Add an
> XXX: before that "we don't support THP writes" comment for good
> measure :)

Sure.

It could be a separate patch which makes add_to_swap clean via
removing page_list argument but I hope Huang take/fold it when he
resend it because it would be more important with THP swap.

Thanks.

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