Re: [RFC 0/6]mm: add new LRU list for MADV_FREE pages

[Johannes Weiner <hannes@cmpxchg.org>]

Hi Shaohua,

On Sun, Jan 29, 2017 at 09:51:17PM -0800, Shaohua Li wrote:
> We are trying to use MADV_FREE in jemalloc. Several issues are found. Without
> solving the issues, jemalloc can't use the MADV_FREE feature.
> - Doesn't support system without swap enabled. Because if swap is off, we can't
>   or can't efficiently age anonymous pages. And since MADV_FREE pages are mixed
>   with other anonymous pages, we can't reclaim MADV_FREE pages. In current
>   implementation, MADV_FREE will fallback to MADV_DONTNEED without swap enabled.
>   But in our environment, a lot of machines don't enable swap. This will prevent
>   our setup using MADV_FREE.
> - Increases memory pressure. page reclaim bias file pages reclaim against
>   anonymous pages. This doesn't make sense for MADV_FREE pages, because those
>   pages could be freed easily and refilled with very slight penality. Even page
>   reclaim doesn't bias file pages, there is still an issue, because MADV_FREE
>   pages and other anonymous pages are mixed together. To reclaim a MADV_FREE
>   page, we probably must scan a lot of other anonymous pages, which is
>   inefficient. In our test, we usually see oom with MADV_FREE enabled and nothing
>   without it.

Fully agreed, the anon LRU is a bad place for these pages.

> For the first two issues, introducing a new LRU list for MADV_FREE pages could
> solve the issues. We can directly reclaim MADV_FREE pages without writting them
> out to swap, so the first issue could be fixed. If only MADV_FREE pages are in
> the new list, page reclaim can easily reclaim such pages without interference
> of file or anonymous pages. The memory pressure issue will disappear.

Do we actually need a new page flag and a special LRU for them? These
pages are basically like clean cache pages at that point. What do you
think about clearing their PG_swapbacked flag on MADV_FREE and moving
them to the inactive file list? The way isolate+putback works should
not even need much modification, something like clear_page_mlock().

When the reclaim scanner finds anon && dirty && !swapbacked, it can
again set PG_swapbacked and goto keep_locked to move the page back
into the anon LRU to get reclaimed according to swapping rules.

> For the third issue, we can add a separate RSS count for MADV_FREE pages. The
> count will be increased in madvise syscall and decreased in page reclaim (eg,
> unmap). One issue is activate_page(). A MADV_FREE page can be promoted to
> active page there. But there isn't mm_struct context at that place. Iterating
> vma there sounds too silly. The patchset don't fix this issue yet. Hopefully
> somebody can share a hint how to fix this issue.

This problem also goes away if we use the file LRUs.