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Date: Thu, 14 Dec 2023 14:23:20 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Chengming Zhou <zhouchengming@bytedance.com>, Nhat Pham
 <nphamcs@gmail.com>, Chris Li <chriscli@google.com>, Johannes Weiner
 <hannes@cmpxchg.org>, Seth Jennings <sjenning@redhat.com>, Dan Streetman
 <ddstreet@ieee.org>, Vitaly Wool <vitaly.wool@konsulko.com>,
 linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 5/5] mm/zswap: cleanup zswap_reclaim_entry()
Message-Id: <20231214142320.f5cf319e619dbb2127c423e9@linux-foundation.org>
In-Reply-To: <CAJD7tkb5cKopA-Lfvtsn7sqgqjRf2kyaMwZhhp6SkveNEwArGw@mail.gmail.com>
References: <20231213-zswap-dstmem-v1-0-896763369d04@bytedance.com>
	<20231213-zswap-dstmem-v1-5-896763369d04@bytedance.com>
	<CAJD7tkb5cKopA-Lfvtsn7sqgqjRf2kyaMwZhhp6SkveNEwArGw@mail.gmail.com>
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On Wed, 13 Dec 2023 17:02:25 -0800 Yosry Ahmed <yosryahmed@google.com> wrote:

> On Tue, Dec 12, 2023 at 8:18 PM Chengming Zhou
> <zhouchengming@bytedance.com> wrote:
> >
> > Also after the common decompress part goes to __zswap_load(), we can
> > cleanup the zswap_reclaim_entry() a little.
> 
> I think you mean zswap_writeback_entry(), same for the commit title.

I updated my copy of the changelog, thanks.

> > -       /*
> > -        * If we get here because the page is already in swapcache, a
> > -        * load may be happening concurrently. It is safe and okay to
> > -        * not free the entry. It is also okay to return !0.
> > -        */
> 
> This comment should be moved above the failure check of
> __read_swap_cache_async() above, not completely removed.

This?

--- a/mm/zswap.c~mm-zswap-cleanup-zswap_reclaim_entry-fix
+++ a/mm/zswap.c
@@ -1457,8 +1457,14 @@ static int zswap_writeback_entry(struct
 	mpol = get_task_policy(current);
 	page = __read_swap_cache_async(swpentry, GFP_KERNEL, mpol,
 				NO_INTERLEAVE_INDEX, &page_was_allocated, true);
-	if (!page)
+	if (!page) {
+		/*
+		 * If we get here because the page is already in swapcache, a
+		 * load may be happening concurrently. It is safe and okay to
+		 * not free the entry. It is also okay to return !0.
+		 */
 		return -ENOMEM;
+	}
 
 	/* Found an existing page, we raced with load/swapin */
 	if (!page_was_allocated) {