Re: [PATCH] mm: vmscan: reset sc->priority on retry

[Roman Gushchin <roman.gushchin@linux.dev>]

On Wed, May 29, 2024 at 08:49:11AM -0700, Shakeel Butt wrote:
> The commit 6be5e186fd65 ("mm: vmscan: restore incremental cgroup
> iteration") added a retry reclaim heuristic to iterate all the cgroups
> before returning an unsuccessful reclaim but missed to reset the
> sc->priority. Let's fix it.
> 
> Reported-and-tested-by: syzbot+17416257cb95200cba44@syzkaller.appspotmail.com
> Fixes: 6be5e186fd65 ("mm: vmscan: restore incremental cgroup iteration")
> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>

Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>

Good catch!

> ---
>  mm/vmscan.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index b9170f767353..731b009a142b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -6317,6 +6317,7 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
>  	 * meaningful forward progress. Avoid false OOMs in this case.
>  	 */
>  	if (!sc->memcg_full_walk) {
> +		sc->priority = initial_priority;
>  		sc->memcg_full_walk = 1;
>  		goto retry;
>  	}
> -- 
> 2.43.0
> 

I wonder if it makes sense to refactor things to be more robust like this:

diff --git a/mm/vmscan.c b/mm/vmscan.c
index d3ae6bf1b65c7..f150e79f736da 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -6246,7 +6246,7 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
        if (!cgroup_reclaim(sc))
                __count_zid_vm_events(ALLOCSTALL, sc->reclaim_idx, 1);

-       do {
+       for (sc->priority = initial_priority; sc->priority >= 0; sc->priority--) {
                if (!sc->proactive)
                        vmpressure_prio(sc->gfp_mask, sc->target_mem_cgroup,
                                        sc->priority);
@@ -6265,7 +6265,7 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
                 */
                if (sc->priority < DEF_PRIORITY - 2)
                        sc->may_writepage = 1;
-       } while (--sc->priority >= 0);
+       }

        last_pgdat = NULL;
        for_each_zone_zonelist_nodemask(zone, z, zonelist, sc->reclaim_idx,
@@ -6318,7 +6318,6 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
         * good, and retry with forcible deactivation if that fails.
         */
        if (sc->skipped_deactivate) {
-               sc->priority = initial_priority;
                sc->force_deactivate = 1;
                sc->skipped_deactivate = 0;
                goto retry;
@@ -6326,7 +6325,6 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,

        /* Untapped cgroup reserves?  Don't OOM, retry. */
        if (sc->memcg_low_skipped) {
-               sc->priority = initial_priority;
                sc->force_deactivate = 0;
                sc->memcg_low_reclaim = 1;
                sc->memcg_low_skipped = 0;