Re: [PATCH 6.6.y 3/3] mm/mglru: fix ineffective protection calculation

[Greg KH <greg@kroah.com>]

On Mon, Jul 29, 2024 at 01:44:34AM -0600, Yu Zhao wrote:
> mem_cgroup_calculate_protection() is not stateless and should only be used
> as part of a top-down tree traversal.  shrink_one() traverses the per-node
> memcg LRU instead of the root_mem_cgroup tree, and therefore it should not
> call mem_cgroup_calculate_protection().
> 
> The existing misuse in shrink_one() can cause ineffective protection of
> sub-trees that are grandchildren of root_mem_cgroup.  Fix it by reusing
> lru_gen_age_node(), which already traverses the root_mem_cgroup tree, to
> calculate the protection.
> 
> Previously lru_gen_age_node() opportunistically skips the first pass,
> i.e., when scan_control->priority is DEF_PRIORITY.  On the second pass,
> lruvec_is_sizable() uses appropriate scan_control->priority, set by
> set_initial_priority() from lru_gen_shrink_node(), to decide whether a
> memcg is too small to reclaim from.
> 
> Now lru_gen_age_node() unconditionally traverses the root_mem_cgroup tree.
> So it should call set_initial_priority() upfront, to make sure
> lruvec_is_sizable() uses appropriate scan_control->priority on the first
> pass.  Otherwise, lruvec_is_reclaimable() can return false negatives and
> result in premature OOM kills when min_ttl_ms is used.
> 
> Link: https://lkml.kernel.org/r/20240712232956.1427127-1-yuzhao@google.com
> Fixes: e4dde56cd208 ("mm: multi-gen LRU: per-node lru_gen_folio lists")
> Signed-off-by: Yu Zhao <yuzhao@google.com>
> Reported-by: T.J. Mercier <tjmercier@google.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> (cherry picked from commit 30d77b7eef019fa4422980806e8b7cdc8674493e)
> ---
>  mm/vmscan.c | 83 ++++++++++++++++++++++++-----------------------------
>  1 file changed, 38 insertions(+), 45 deletions(-)

Now queued up, thanks.

greg k-h