linux-kernel - vmscan.c: Reclaim unevictable pages.

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Date:   Sat, 6 Apr 2019 11:29:27 +0530
From:   Pankaj Suryawanshi <pankajssuryawanshi@...il.com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kernelnewbies@...nelnewbies.org, vbabka@...e.cz, mhocko@...nel.org,
        minchan@...nel.org
Subject: vmscan.c: Reclaim unevictable pages.

Hello ,

shrink_page_list() returns , number of pages reclaimed, when pages is
unevictable it returns VM_BUG_ON_PAGE(PageLRU(page) ||
PageUnevicatble(page),page);

We can add the unevictable pages in reclaim list in
shrink_page_list(), return total number of reclaim pages including
unevictable pages, let the caller handle unevictable pages.

I think the problem is shrink_page_list is awkard. If page is
unevictable it goto activate_locked->keep_locked->keep lables, keep
lable list_add the unevictable pages and throw the VM_BUG instead of
passing it to caller while it relies on caller for
non-reclaimed-non-unevictable  page's putback.
I think we can make it consistent so that shrink_page_list could
return non-reclaimed pages via page_list and caller can handle it. As
an advance, it could try to migrate mlocked pages without retrial.

Below is the issue i observed of CMA_ALLOC of large size buffer :
(Kernel version - 4.14.65 With Android Pie.

[   24.718792] page dumped because: VM_BUG_ON_PAGE(PageLRU(page) ||
PageUnevictable(page))
[   24.726949] page->mem_cgroup:bd008c00
[   24.730693] ------------[ cut here ]------------
[   24.735304] kernel BUG at mm/vmscan.c:1350!
[   24.739478] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM

Below is the patch which solved this issue :

diff --git a/mm/vmscan.c b/mm/vmscan.c
index be56e2e..12ac353 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -998,7 +998,7 @@ static unsigned long shrink_page_list(struct
list_head *page_list,
                sc->nr_scanned++;

                if (unlikely(!page_evictable(page)))
-                       goto activate_locked;
+                      goto cull_mlocked;

                if (!sc->may_unmap && page_mapped(page))
                        goto keep_locked;
@@ -1331,7 +1331,12 @@ static unsigned long shrink_page_list(struct
list_head *page_list,
                } else
                        list_add(&page->lru, &free_pages);
                continue;
-
+cull_mlocked:
+                if (PageSwapCache(page))
+                        try_to_free_swap(page);
+                unlock_page(page);
+                list_add(&page->lru, &ret_pages);
+                continue;
 activate_locked:
                /* Not a candidate for swapping, so reclaim swap space. */
                if (PageSwapCache(page) && (mem_cgroup_swap_full(page) ||

It fixes the below issue.

1. Large size buffer allocation using cma_alloc successful with
unevictable pages.

cma_alloc of current kernel will fail due to unevictable page

Please let me know if anything i am missing.

Regards,
Pankaj