Michal Hocko <mhocko@xxxxxxxxxx> writes:
> 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.
I can remove all these places. Only keep the helpful places.
> 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.
Yes. The following piece isn't lazy free specific. We can keep use PageSwapBacked().
@@ -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;
And the following piece is lazy free specific. I think it helps.
@@ -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;
> 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.
Yes. I think it's good to add document for PG_swapbacked definition.
> That being said I think that the patch is not helpful enough. I would
> much rather see a simply documentation update.
