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SubjectRe: [PATCH for mmotm 0414] vmscan,memcg: reintroduce sc->may_swap
Hi KOSAKI-san,

On Sat, Apr 18, 2009 at 03:26:02PM +0900, KOSAKI Motohiro wrote:
> Subject: vmscan,memcg: reintroduce sc->may_swap
>
> vmscan-rename-scmay_swap-to-may_unmap.patch removed may_swap flag,
> but memcg had used it as a flag for "we need to use swap?", as the
> name indicate.
>
> And in current implementation, memcg cannot reclaim mapped file caches
> when mem+swap hits the limit.
>
> re-introduce may_swap flag and handle it at get_scan_ratio().
> This patch doesn't influence any scan_control users other than memcg.
>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> --
> mm/vmscan.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
> Index: b/mm/vmscan.c
> ===================================================================
> --- a/mm/vmscan.c 2009-04-16 21:25:41.000000000 +0900
> +++ b/mm/vmscan.c 2009-04-16 21:56:54.000000000 +0900
> @@ -64,6 +64,9 @@ struct scan_control {
> /* Can mapped pages be reclaimed? */
> int may_unmap;
>
> + /* Can pages be swapped as part of reclaim? */
> + int may_swap;
> +
> /* This context's SWAP_CLUSTER_MAX. If freeing memory for
> * suspend, we effectively ignore SWAP_CLUSTER_MAX.
> * In this context, it doesn't matter that we scan the
> @@ -1387,7 +1390,7 @@ static void get_scan_ratio(struct zone *
> struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);
>
> /* If we have no swap space, do not bother scanning anon pages. */
> - if (nr_swap_pages <= 0) {
> + if (!sc->may_swap || (nr_swap_pages <= 0)) {

I wonder if may_swap is a good name for this effect. See below the
__zone_reclaim() comments.

> percent[0] = 0;
> percent[1] = 100;
> return;
> @@ -1704,6 +1707,7 @@ unsigned long try_to_free_pages(struct z
> .may_writepage = !laptop_mode,
> .swap_cluster_max = SWAP_CLUSTER_MAX,
> .may_unmap = 1,
> + .may_swap = 1,
> .swappiness = vm_swappiness,
> .order = order,
> .mem_cgroup = NULL,
> @@ -1724,6 +1728,7 @@ unsigned long try_to_free_mem_cgroup_pag
> struct scan_control sc = {
> .may_writepage = !laptop_mode,
> .may_unmap = 1,
> + .may_swap = 1,
> .swap_cluster_max = SWAP_CLUSTER_MAX,
> .swappiness = swappiness,
> .order = 0,
> @@ -1734,7 +1739,7 @@ unsigned long try_to_free_mem_cgroup_pag
> struct zonelist *zonelist;
>
> if (noswap)
> - sc.may_unmap = 0;
> + sc.may_swap = 0;

Can this be directly initialized?

struct scan_control sc = {
...
.may_swap = !noswap,
...
};

> sc.gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
> (GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK);
> @@ -1774,6 +1779,7 @@ static unsigned long balance_pgdat(pg_da
> struct scan_control sc = {
> .gfp_mask = GFP_KERNEL,
> .may_unmap = 1,
> + .may_swap = 1,
> .swap_cluster_max = SWAP_CLUSTER_MAX,
> .swappiness = vm_swappiness,
> .order = order,
> @@ -2120,6 +2126,7 @@ unsigned long shrink_all_memory(unsigned
> struct scan_control sc = {
> .gfp_mask = GFP_KERNEL,
> .may_unmap = 0,
> + .may_swap = 1,

shrink_all_memory() is not a user of shrink_zone() -> get_scan_ratio()
and therefor not affected by this flag. I think it's better not to
set it here (just like sc->swappiness).

> .may_writepage = 1,
> .isolate_pages = isolate_pages_global,
> };
> @@ -2304,6 +2311,7 @@ static int __zone_reclaim(struct zone *z
> struct scan_control sc = {
> .may_writepage = !!(zone_reclaim_mode & RECLAIM_WRITE),
> .may_unmap = !!(zone_reclaim_mode & RECLAIM_SWAP),
> + .may_swap = 1,

Shouldn't this be set to !!(zone_reclaim_mode & RECLAIM_SWAP) as well?

With set to 1, zone_reclaim() will also reclaim unmapped swap cache
pages (without swapping) and it might be desirable to do that. But
then may_swap is a confusing name. may_anon? may_scan_anon?
scan_anon?

Hannes


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