Re: [PATCH v2 mm-hotfixes] mm/zswap: fix inconsistent charging when zswap_store_page() fails

[Yosry Ahmed <yosry.ahmed@linux.dev>]

On Wed, Jan 29, 2025 at 03:55:07AM +0900, Hyeonggon Yoo wrote:
> Commit b7c0ccdfbafd ("mm: zswap: support large folios in zswap_store()")
> skips charging any zswapped base pages when it failed to zswap the entire
> folio.
> 
> However, when some base pages are zswapped but it failed to zswap
> the entire folio, the zswap operation is rolled back.
> When freeing zswap entries for those pages, zswap_entry_free() uncharges
> the pages that were not previously charged, causing zswap charging to
> become inconsistent.
> 
> This inconsistency triggers two warnings with following steps:
>   # On a machine with 64GiB of RAM and 36GiB of zswap
>   $ stress-ng --bigheap 2 # wait until the OOM-killer kills stress-ng
>   $ sudo reboot
> 
>   Two warnings are:
>     in mm/memcontrol.c:163, function obj_cgroup_release():
>       WARN_ON_ONCE(nr_bytes & (PAGE_SIZE - 1));
> 
>     in mm/page_counter.c:60, function page_counter_cancel():
>       if (WARN_ONCE(new < 0, "page_counter underflow: %ld nr_pages=%lu\n",
> 	  new, nr_pages))
> 
> While objcg events should only be accounted for when the entire folio is
> zswapped, objcg charging should be performed regardlessly.
> Fix accordingly.
> 
> After resolving the inconsistency, these warnings disappear.
> 
> Fixes: b7c0ccdfbafd ("mm: zswap: support large folios in zswap_store()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> ---
> 
> v1->v2:
> 
>  Fixed objcg events being accounted for on zswap failure.
> 
>  Fixed the incorrect description. I misunderstood that the base pages are
>  going to be stored in zswap, but their zswap entries are freed immediately.
> 
>  Added a comment on why it charges pages that are going to be removed
>  from zswap.
> 
>  mm/zswap.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 6504174fbc6a..10b30ac46deb 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1568,20 +1568,26 @@ bool zswap_store(struct folio *folio)
>  
>  		bytes = zswap_store_page(page, objcg, pool);
>  		if (bytes < 0)
> -			goto put_pool;
> +			goto charge_zswap;
>  		compressed_bytes += bytes;
>  	}
>  
> -	if (objcg) {
> -		obj_cgroup_charge_zswap(objcg, compressed_bytes);
> +	if (objcg)
>  		count_objcg_events(objcg, ZSWPOUT, nr_pages);
> -	}
>  
>  	atomic_long_add(nr_pages, &zswap_stored_pages);
>  	count_vm_events(ZSWPOUT, nr_pages);
>  
>  	ret = true;
>  
> +charge_zswap:
> +	/*
> +	 * Charge zswapped pages even when it failed to zswap the entire folio,
> +	 * because zswap_entry_free() will uncharge them anyway.
> +	 * Otherwise zswap charging will become inconsistent.
> +	 */
> +	if (objcg)
> +		obj_cgroup_charge_zswap(objcg, compressed_bytes);

Thanks for fixing this!

Having to charge just to uncharge right after is annoying. Ideally we'd
just clear entry->objcg if we fail before charging, but we don't have a
direct reference to the entries here and another tree lookup is not
ideal either.

I guess we may be able to improve this handling once [1] lands, as we
can move the charging logic into zswap_store_folio() where we'd have
access to the entries.

For now, would the control flow be easier if we move the charge ahead of
the zswap_store_page() loop instead? There is an existing if (objcg)
block there as well.

[1]https://lore.kernel.org/linux-mm/20241221063119.29140-12-kanchana.p.sridhar@intel.com/

>  put_pool:
>  	zswap_pool_put(pool);
>  put_objcg:
> -- 
> 2.47.1
> 
>