On Mon 09-03-20 09:55:38, David Hildenbrand wrote:
> On 09.03.20 03:17, Huang, Ying wrote:
[...]
> > @@ -1235,7 +1234,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> > * Try to allocate it some swap space here.
> > * Lazyfree page could be freed directly
> > */
> > - if (PageAnon(page) && PageSwapBacked(page)) {
> > + if (PageAnon(page) && !__PageLazyFree(page)) {
> > if (!PageSwapCache(page)) {
> > if (!(sc->gfp_mask & __GFP_IO))
> > goto keep_locked;
> > @@ -1411,7 +1410,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> > }
> > }
> >
> > - if (PageAnon(page) && !PageSwapBacked(page)) {
> > + if (PageLazyFree(page)) {
> > /* follow __remove_mapping for reference */
> > if (!page_ref_freeze(page, 1))
> > goto keep_locked;
> >
>
> I still prefer something like
>
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index fd6d4670ccc3..7538501230bd 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -63,6 +63,10 @@
> * page_waitqueue(page) is a wait queue of all tasks waiting for the page
> * to become unlocked.
> *
> + * PG_swapbacked used with anonymous pages (PageAnon()) indicates that a
> + * page is backed by swap. Anonymous pages without PG_swapbacked are
> + * pages that can be lazily freed (e.g., MADV_FREE) on demand.
> + *
> * PG_uptodate tells whether the page's contents is valid. When a read
> * completes, the page becomes uptodate, unless a disk I/O error happened.
> *
>
> and really don't like the use of !__PageLazyFree() instead of PageSwapBacked().
I have to say that I do not have a strong opinion about helper
functions. In general I tend to be against adding them unless there is a
very good reason for them. This particular patch is in a gray zone a bit.
There are few places which are easier to follow but others sound like,
we have a hammer let's use it. E.g. shrink_page_list path above. There
is a clear comment explaining PageAnon && PageSwapBacked check being
LazyFree related but do I have to know that this is LazyFree path? I
believe that seeing PageSwapBacked has a more meaning to me because it
tells me that anonymous pages without a backing store doesn't really
need swap entry. This happens to be Lazy free related today but with a
heavy overloading of our flags this might differ in the future. You have
effectively made a more generic description more specific without a very
good reason.
On the other hand having PG_swapbacked description in page-flags.h above
gives a very useful information which was previously hidden at the
definition so this is a clear improvement.
That being said I think that the patch is not helpful enough. I would
much rather see a simply documentation update.
--
Michal Hocko
SUSE Labs
