Re: [PATCH -mm] memcg: fix handling of shmem migration

[KAMEZAWA Hiroyuki]

On Wed, 17 Sep 2008 13:31:49 +0900
Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:

> PG_swapbacked flag of newpage should be set(if needed) before
> mem_cgroup_prepare_migration, because mem_cgroup_charge_common
> checks the flag and determines whether it sets PAGE_CGROUP_FLAG_FILE or not.
> 
> Before this patch, if migrating shmem/tmpfs pages, newpage would be
> charged with PAGE_CGROUP_FLAG_FILE set, while oldpage has been charged
> without the flag.
> 
Nice catch !
Thank you. 

Hmm, should I add MEM_CGROUP_CHARGE_TYPE_SHMEM rather than
setting flag to newpage ?

Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>


> The problem here is mem_cgroup_move_lists doesn't clear(or set)
> the PAGE_CGROUP_FLAG_FILE flag, so pc->flags of the newpage
> remains PAGE_CGROUP_FLAG_FILE set even when the pc is moved to
> another lru(anon) by mem_cgroup_move_lists. And this leads to
> incorrect MEM_CGROUP_ZSTAT.
> (In my test, I see an underflow of MEM_CGROUP_ZSTAT(active_file).
> As a result, mem_cgroup_calc_reclaim returns very huge number and
> causes soft lockup on page reclaim.)
> 
> I'm not sure if mem_cgroup_move_lists should handle PAGE_CGROUP_FLAG_FILE
> or not(I suppose it should be used to move between active <-> inactive,
> not anon <-> file), I moved SetPageSwapBacked(newpage) before
> mem_cgroup_prepare_migration.
> 
> 
> Signed-off-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
> ---
>  mm/migrate.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 577d481..7343463 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -586,8 +586,6 @@ static int move_to_new_page(struct page *newpage, struct page *page)
>  	/* Prepare mapping for the new page.*/
>  	newpage->index = page->index;
>  	newpage->mapping = page->mapping;
> -	if (PageSwapBacked(page))
> -		SetPageSwapBacked(newpage);
>  
>  	mapping = page_mapping(page);
>  	if (!mapping)
> @@ -636,6 +634,8 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  		goto move_newpage;
>  	}
>  
> +	if (PageSwapBacked(page))
> +		SetPageSwapBacked(newpage);
>  	charge = mem_cgroup_prepare_migration(page, newpage);
>  	if (charge == -ENOMEM) {
>  		rc = -ENOMEM;
> 

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