linux-kernel - Re: [PATCH 05/18] MM: reclaim mustn't enter FS for SWP_FS

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Date:   Tue, 21 Dec 2021 00:43:18 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     NeilBrown <neilb@...e.de>
Cc:     Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Chuck Lever <chuck.lever@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        Christoph Hellwig <hch@...radead.org>,
        David Howells <dhowells@...hat.com>, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/18] MM: reclaim mustn't enter FS for SWP_FS_OPS
 swap-space

On Fri, Dec 17, 2021 at 10:48:22AM +1100, NeilBrown wrote:
> +static bool test_may_enter_fs(struct page *page, gfp_t gfp_mask)
> +{
> +	if (gfp_mask & __GFP_FS)
> +		return true;
> +	if (!PageSwapCache(page) || !(gfp_mask & __GFP_IO))
> +		return false;
> +	/* We can "enter_fs" for swap-cache with only __GFP_IO
> +	 * providing this isn't SWP_FS_OPS.
> +	 * ->flags can be updated non-atomicially (scan_swap_map_slots),
> +	 * but that will never affect SWP_FS_OPS, so the data_race
> +	 * is safe.
> +	 */
> +	return !data_race(page_swap_info(page)->flags & SWP_FS_OPS);

Nit: the normal kernel comment style uses an empty

	/*

line to start the comment.

> @@ -1514,8 +1529,7 @@ static unsigned int shrink_page_list(struct list_head *page_list,
>  		if (!sc->may_unmap && page_mapped(page))
>  			goto keep_locked;
>  
> -		may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
> -			(PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));
> +		may_enter_fs = test_may_enter_fs(page, sc->gfp_mask);
>  
>  		/*
>  		 * The number of dirty pages determines if a node is marked
> @@ -1683,7 +1697,8 @@ static unsigned int shrink_page_list(struct list_head *page_list,
>  						goto activate_locked_split;
>  				}
>  
> -				may_enter_fs = true;
> +				may_enter_fs = test_may_enter_fs(page,
> +								 sc->gfp_mask);

Now that may_enter_fs is always reset using test_may_enter_fs, do we
even need the local variable?