Re: [PATCH 2/4] nvdimm/pmem: avoid HWPoison flag updates for clean pages

["Gupta, Pankaj" <pankaj.gupta@amd.com>]

> pmem_mkpage_present() is called after persistent-memory poison has been
> cleared. It only needs to clear PG_hwpoison and undo the MCE nospec state
> for PFNs that are still marked poisoned.
>
> For clean PFNs, test_and_clear_pmem_poison() has no semantic effect, but it
> still performs an atomic clear operation against struct page flags. That is
> unnecessary today and conflicts with the FS-DAX vmemmap optimization added
> later in the series, where clean PFNs may be backed by shared read-only
> metadata.
>
> Check PageHWPoison() first so only poisoned PFNs update the page state.
> Poison recovery keeps the same behavior, while clean PFNs no longer require
> writable per-PFN metadata.
>
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>

Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>

> ---
>   drivers/nvdimm/pmem.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> index 30a51c365ce8..b14f75daeda7 100644
> --- a/drivers/nvdimm/pmem.c
> +++ b/drivers/nvdimm/pmem.c
> @@ -80,7 +80,7 @@ static void pmem_mkpage_present(struct pmem_device *pmem, phys_addr_t offset,
>   		 * here since we're in the driver I/O path and
>   		 * outstanding I/O requests pin the dev_pagemap.
>   		 */
> -		if (test_and_clear_pmem_poison(page))
> +		if (PageHWPoison(page) && test_and_clear_pmem_poison(page))
>   			clear_mce_nospec(pfn);
>   	}
>   }