Re: [PATCH v3 3/9] mm: slightly clarify KSM logic in do_swap_page()

[Vlastimil Babka <vbabka@suse.cz>]

On 1/31/22 17:29, David Hildenbrand wrote:
> Let's make it clearer that KSM might only have to copy a page
> in case we have a page in the swapcache, not if we allocated a fresh
> page and bypassed the swapcache. While at it, add a comment why this is
> usually necessary and merge the two swapcache conditions.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>


> ---
>  mm/memory.c | 38 +++++++++++++++++++++++---------------
>  1 file changed, 23 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 923165b4c27e..3c91294cca98 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3615,21 +3615,29 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  		goto out_release;
>  	}
>  
> -	/*
> -	 * Make sure try_to_free_swap or reuse_swap_page or swapoff did not
> -	 * release the swapcache from under us.  The page pin, and pte_same
> -	 * test below, are not enough to exclude that.  Even if it is still
> -	 * swapcache, we need to check that the page's swap has not changed.
> -	 */
> -	if (unlikely((!PageSwapCache(page) ||
> -			page_private(page) != entry.val)) && swapcache)
> -		goto out_page;
> -
> -	page = ksm_might_need_to_copy(page, vma, vmf->address);
> -	if (unlikely(!page)) {
> -		ret = VM_FAULT_OOM;
> -		page = swapcache;
> -		goto out_page;
> +	if (swapcache) {
> +		/*
> +		 * Make sure try_to_free_swap or reuse_swap_page or swapoff did
> +		 * not release the swapcache from under us.  The page pin, and
> +		 * pte_same test below, are not enough to exclude that.  Even if
> +		 * it is still swapcache, we need to check that the page's swap
> +		 * has not changed.
> +		 */
> +		if (unlikely(!PageSwapCache(page) ||
> +			     page_private(page) != entry.val))
> +			goto out_page;
> +
> +		/*
> +		 * KSM sometimes has to copy on read faults, for example, if
> +		 * page->index of !PageKSM() pages would be nonlinear inside the
> +		 * anon VMA -- PageKSM() is lost on actual swapout.
> +		 */
> +		page = ksm_might_need_to_copy(page, vma, vmf->address);
> +		if (unlikely(!page)) {
> +			ret = VM_FAULT_OOM;
> +			page = swapcache;
> +			goto out_page;
> +		}
>  	}
>  
>  	cgroup_throttle_swaprate(page, GFP_KERNEL);