Re: [PATCH 05/18] mm: memcontrol: convert page cache to a new mem_cgroup_charge() API

[Hugh Dickins <hughd@google.com>]

On Mon, 11 May 2020, Johannes Weiner wrote:
> 
> Since commit b56a2d8af914 ("mm: rid swapoff of quadratic complexity"),
> shmem_unuse_inode() doesn't have its own copy anymore - it uses
> shmem_swapin_page().
> 
> However, that commit appears to have made shmem's private call to
> delete_from_swap_cache() obsolete as well. Whereas before this change
> we fully relied on shmem_unuse() to find and clear a shmem swap entry
> and its swapcache page, we now only need it to clean out shmem's
> private state in the inode, as it's followed by a loop over all
> remaining swap slots, calling try_to_free_swap() on stragglers.

Great, you've looked deeper into the current situation than I had.

> 
> Unless I missed something, it's still merely an optimization, and we
> can delete it for simplicity:

Yes, nice ---s, simpler code, and a good idea to separate it out
as a precursor: thanks, Hannes.

> 
> ---
> 
> From fc9dcaf68c8b54baf365cd670fb5780c7f0d243f Mon Sep 17 00:00:00 2001
> From: Johannes Weiner <hannes@cmpxchg.org>
> Date: Mon, 11 May 2020 12:59:08 -0400
> Subject: [PATCH] mm: shmem: remove rare optimization when swapin races with
>  hole punching
> 
> Commit 215c02bc33bb ("tmpfs: fix shmem_getpage_gfp() VM_BUG_ON")
> recognized that hole punching can race with swapin and removed the
> BUG_ON() for a truncated entry from the swapin path.
> 
> The patch also added a swapcache deletion to optimize this rare case:
> Since swapin has the page locked, and free_swap_and_cache() merely
> trylocks, this situation can leave the page stranded in
> swapcache. Usually, page reclaim picks up stale swapcache pages, and
> the race can happen at any other time when the page is locked. (The
> same happens for non-shmem swapin racing with page table zapping.) The
> thinking here was: we already observed the race and we have the page
> locked, we may as well do the cleanup instead of waiting for reclaim.
> 
> However, this optimization complicates the next patch which moves the
> cgroup charging code around. As this is just a minor speedup for a
> race condition that is so rare that it required a fuzzer to trigger
> the original BUG_ON(), it's no longer worth the complications.
> 
> Suggested-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Acked-by: Hugh Dickins <hughd@google.com>
(if one is allowed to suggest and to ack)

> ---
>  mm/shmem.c | 25 +++++++------------------
>  1 file changed, 7 insertions(+), 18 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index d505b6cce4ab..729bbb3513cd 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1665,27 +1665,16 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>  	}
>  
>  	error = mem_cgroup_try_charge_delay(page, charge_mm, gfp, &memcg);
> -	if (!error) {
> -		error = shmem_add_to_page_cache(page, mapping, index,
> -						swp_to_radix_entry(swap), gfp);
> -		/*
> -		 * We already confirmed swap under page lock, and make
> -		 * no memory allocation here, so usually no possibility
> -		 * of error; but free_swap_and_cache() only trylocks a
> -		 * page, so it is just possible that the entry has been
> -		 * truncated or holepunched since swap was confirmed.
> -		 * shmem_undo_range() will have done some of the
> -		 * unaccounting, now delete_from_swap_cache() will do
> -		 * the rest.
> -		 */
> -		if (error) {
> -			mem_cgroup_cancel_charge(page, memcg);
> -			delete_from_swap_cache(page);
> -		}
> -	}
>  	if (error)
>  		goto failed;
>  
> +	error = shmem_add_to_page_cache(page, mapping, index,
> +					swp_to_radix_entry(swap), gfp);
> +	if (error) {
> +		mem_cgroup_cancel_charge(page, memcg);
> +		goto failed;
> +	}
> +
>  	mem_cgroup_commit_charge(page, memcg, true);
>  
>  	spin_lock_irq(&info->lock);
> -- 
> 2.26.2
> 
>