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Date:   Thu, 18 Mar 2021 15:01:25 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Hugh Dickins <hughd@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <guro@fb.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Minchan Kim <minchan@kernel.org>, cgroups@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Heiko Carstens <hca@linux.ibm.com>
Subject: Re: [PATCH] memcg: set page->private before calling swap_readpage
Message-ID: <YFNdNeczjnmF55bm@dhcp22.suse.cz>
References: <20210318015959.2986837-1-shakeelb@google.com>
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On Wed 17-03-21 18:59:59, Shakeel Butt wrote:
> The function swap_readpage() (and other functions it call) extracts swap
> entry from page->private. However for SWP_SYNCHRONOUS_IO, the kernel
> skips the swapcache and thus we need to manually set the page->private
> with the swap entry before calling swap_readpage().

One thing that is not really clear to me is whether/why this is only
needed with your patch. Can you expand a bit on that please? Maybe I am
just missing something obvious but I just do not see any connection.

> Signed-off-by: Shakeel Butt <shakeelb@google.com>
> Reported-by: Heiko Carstens <hca@linux.ibm.com>
> ---
> 
> Andrew, please squash this into "memcg: charge before adding to
> swapcache on swapin" patch.
> 
>  mm/memory.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index aefd158ae1ea..b6f3410b5902 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3324,7 +3324,11 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  					workingset_refault(page, shadow);
>  
>  				lru_cache_add(page);
> +
> +				/* To provide entry to swap_readpage() */
> +				set_page_private(page, entry.val);
>  				swap_readpage(page, true);
> +				set_page_private(page, 0);
>  			}
>  		} else {
>  			page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE,
> -- 
> 2.31.0.rc2.261.g7f71774620-goog
> 

-- 
Michal Hocko
SUSE Labs