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Date:   Fri, 11 Oct 2019 14:32:31 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Minchan Kim <minchan@kernel.org>, linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Sahkeel Butt <shakeelb@google.com>,
        Minchan Kim <minchan@google.com>
Subject: Re: [PATCH] mm: annotate refault stalls from swap_readpage
Message-Id: <20191011143231.e338e0ef337492e83233ad39@linux-foundation.org>
In-Reply-To: <20191010191747.GA31673@cmpxchg.org>
References: <20191010152134.38545-1-minchan@kernel.org>
        <20191010191747.GA31673@cmpxchg.org>
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On Thu, 10 Oct 2019 15:17:47 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:

> On Thu, Oct 10, 2019 at 08:21:34AM -0700, Minchan Kim wrote:
> > From: Minchan Kim <minchan@google.com>
> > 
> > If block device supports rw_page operation, it doesn't submit bio
> > so annotation in submit_bio for refault stall doesn't work.
> > It happens with zram in android, especially swap read path which
> > could consume CPU cycle for decompress. It is also a problem for
> > zswap which uses frontswap.
> > 
> > Annotate swap_readpage() to account the synchronous IO overhead
> > to prevent underreport memory pressure.
> > 
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Signed-off-by: Minchan Kim <minchan@google.com>
> 
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> 
> Can you please add a comment to the caller? Lifted from submit_bio():
> 
> 	/*
> 	 * Count submission time as memory stall. When the device is
> 	 * congested, or the submitting cgroup IO-throttled,
> 	 * submission can be a significant part of overall IO time.
> 	 */

This?

--- a/mm/page_io.c~mm-annotate-refault-stalls-from-swap_readpage-fix
+++ a/mm/page_io.c
@@ -361,6 +361,11 @@ int swap_readpage(struct page *page, boo
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 	VM_BUG_ON_PAGE(PageUptodate(page), page);
 
+	/*
+	 * Count submission time as memory stall. When the device is congested,
+	 * or the submitting cgroup IO-throttled, submission can be a
+	 * significant part of overall IO time.
+	 */
 	psi_memstall_enter(&pflags);
 
 	if (frontswap_load(page) == 0) {
_