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Date:   Mon, 22 Jul 2019 15:26:07 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     linux-mm@kvack.org, linux-btrfs@vger.kernel.org,
        linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] psi: annotate refault stalls from IO submission
Message-Id: <20190722152607.dd175a9d517a5f6af06a8bdc@linux-foundation.org>
In-Reply-To: <20190722201337.19180-1-hannes@cmpxchg.org>
References: <20190722201337.19180-1-hannes@cmpxchg.org>
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On Mon, 22 Jul 2019 16:13:37 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:

> psi tracks the time tasks wait for refaulting pages to become
> uptodate, but it does not track the time spent submitting the IO. The
> submission part can be significant if backing storage is contended or
> when cgroup throttling (io.latency) is in effect - a lot of time is
> spent in submit_bio(). In that case, we underreport memory pressure.

It's a somewhat broad patch.  How significant is this problem in the
real world?  Can we be confident that the end-user benefit is worth the
code changes?

> Annotate the submit_bio() paths (or the indirection through readpage)
> for refaults and swapin to get proper psi coverage of delays there.
> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  fs/btrfs/extent_io.c | 14 ++++++++++++--
>  fs/ext4/readpage.c   |  9 +++++++++
>  fs/f2fs/data.c       |  8 ++++++++
>  fs/mpage.c           |  9 +++++++++
>  mm/filemap.c         | 20 ++++++++++++++++++++
>  mm/page_io.c         | 11 ++++++++---
>  mm/readahead.c       | 24 +++++++++++++++++++++++-

We touch three filesystems.  Why these three?  Are all other
filesystems OK or will they need work as well?

> ...
>
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
>
> ...
>
> @@ -2753,11 +2763,14 @@ static struct page *do_read_cache_page(struct address_space *mapping,
>  				void *data,
>  				gfp_t gfp)
>  {
> +	bool refault = false;
>  	struct page *page;
>  	int err;
>  repeat:
>  	page = find_get_page(mapping, index);
>  	if (!page) {
> +		unsigned long pflags;
> +

That was a bit odd.  This?

--- a/mm/filemap.c~psi-annotate-refault-stalls-from-io-submission-fix
+++ a/mm/filemap.c
@@ -2815,12 +2815,12 @@ static struct page *do_read_cache_page(s
 				void *data,
 				gfp_t gfp)
 {
-	bool refault = false;
 	struct page *page;
 	int err;
 repeat:
 	page = find_get_page(mapping, index);
 	if (!page) {
+		bool refault = false;
 		unsigned long pflags;
 
 		page = __page_cache_alloc(gfp);
_