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    SubjectRe: vmscan: Reclaim unevictable pages
    Date
    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 of CMA_ALLOC of large size buffer : (Kernel version - 4.14.65 (On Android pie [ARM])).

    [ 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



    From: Pankaj Suryawanshi
    Sent: 18 March 2019 13:15:22
    To: Kirill Tkhai; Michal Hocko; aneesh.kumar@linux.ibm.com
    Cc: linux-kernel@vger.kernel.org; minchan@kernel.org; linux-mm@kvack.org; khandual@linux.vnet.ibm.com; vbabka@suse.cz
    Subject: Re: Re: [External] Re: vmscan: Reclaim unevictable pages



    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 pages.

    Solved the below issue of cma_alloc

    -----------------------------------------------------------------------------------------------------------------------------------------
    [ 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
    -----------------------------------------------------------------------------------------------------------------------------------------

    From: Pankaj Suryawanshi
    Sent: 15 March 2019 15:41:57
    To: Kirill Tkhai; Michal Hocko; aneesh.kumar@linux.ibm.com
    Cc: linux-kernel@vger.kernel.org; minchan@kernel.org; linux-mm@kvack.org; khandual@linux.vnet.ibm.com; hillf.zj@alibaba-inc.com; vbabka@suse.cz
    Subject: Re: Re: [External] Re: vmscan: Reclaim unevictable pages



    [ cc Aneesh kumar, Anshuman, Hillf, Vlastimil]

    From: Pankaj Suryawanshi
    Sent: 15 March 2019 11:35:05
    To: Kirill Tkhai; Michal Hocko
    Cc: linux-kernel@vger.kernel.org; minchan@kernel.org; linux-mm@kvack.org
    Subject: Re: Re: [External] Re: vmscan: Reclaim unevictable pages



    [ cc linux-mm ]


    From: Pankaj Suryawanshi
    Sent: 14 March 2019 19:14:40
    To: Kirill Tkhai; Michal Hocko
    Cc: linux-kernel@vger.kernel.org; minchan@kernel.org
    Subject: Re: Re: [External] Re: vmscan: Reclaim unevictable pages



    Hello ,

    Please ignore the curly braces, they are just for debugging.

    Below is the updated patch.


    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) ||


    Regards,
    Pankaj


    From: Kirill Tkhai <ktkhai@virtuozzo.com>
    Sent: 14 March 2019 14:55:34
    To: Pankaj Suryawanshi; Michal Hocko
    Cc: linux-kernel@vger.kernel.org; minchan@kernel.org
    Subject: Re: Re: [External] Re: vmscan: Reclaim unevictable pages


    On 14.03.2019 11:52, Pankaj Suryawanshi wrote:
    >
    > I am using kernel version 4.14.65 (on Android pie [ARM]).
    >
    > No additional patches applied on top of vanilla.(Core MM).
    >
    > If I change in the vmscan.c as below patch, it will work.

    Sorry, but 4.14.65 does not have braces around trylock_page(),
    like in your patch below.

    See https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/mm/vmscan.c?h=v4.14.65

    [...]

    >> diff --git a/mm/vmscan.c b/mm/vmscan.c
    >> index be56e2e..2e51edc 100644
    >> --- a/mm/vmscan.c
    >> +++ b/mm/vmscan.c
    >> @@ -990,15 +990,17 @@ static unsigned long shrink_page_list(struct list_head *page_list,
    >> page = lru_to_page(page_list);
    >> list_del(&page->lru);
    >>
    >> if (!trylock_page(page)) {
    >> goto keep;
    >> }

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