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Wed, 24 Jun 2026 04:55:24 -0700 (PDT) Message-ID: <0916e673-861f-b472-7417-afbffbcc98ad@gmail.com> Date: Wed, 24 Jun 2026 19:55:14 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.15.0 Subject: Re: [PATCH v4 2/5] mm/zswap: Factor writeback loop out of shrink_worker() To: Yosry Ahmed Cc: akpm@linux-foundation.org, tj@kernel.org, hannes@cmpxchg.org, shakeel.butt@linux.dev, mhocko@kernel.org, mkoutny@suse.com, nphamcs@gmail.com, chengming.zhou@linux.dev, muchun.song@linux.dev, roman.gushchin@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Hao Jia References: <20260618044857.69439-1-jiahao.kernel@gmail.com> <20260618044857.69439-3-jiahao.kernel@gmail.com> From: Hao Jia In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: pattej8hs5aakn9csgkjb6n6yub1owok X-Rspam-User: X-Rspamd-Queue-Id: E38B540007 X-Rspamd-Server: rspam02 X-HE-Tag: 1782302125-336872 X-HE-Meta: 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 LTbbKB3v owXwBBjff4MCPS8nRCBiXBIPyefVd0TPzKupVVsNCej89/HJ0j6riisj+86fS685YkukYe6amIISKkCBFB67bTY6Qj5XsFiW6755/f/voZyafxkhOhneGjAj5FpzxSiEvJKDhJNyH/fpAkewqaVd0G1HDfn2615TmVOL9YM/JovJSGFYPNySI3XZ2cAJDV/3G8cLZcOT2LIx74PgyB0rssCddMKapLvYyulWrunLjFkbOe6QuBQGQ+TlcC+Yu3eaJkCxLsAYfaK0NhGhHC1ikOpUC4AuJRjH12uY1BqYxNMHF+iJxQl/tlDfxdwWzyZk4f99EF+djViH8wKYiVDWOSd6UOZ+o4sptLcEaQqORXNzI6ffVE6aXjRmwEXzGNPljHs0/I8FIKvhz0asaD81qogidv7WVSD1Lx7S7kqXg9ojQYwz+FlFDxeQWr3xHL1NoLIKmKPpCfl4XqfWoRaA6cDSzr7m5a6rpXAr+z1B3veyWHoE= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2026/6/23 07:36, Yosry Ahmed wrote: >> +/* >> + * Walk the memcg tree and write back zswap pages until the >> + * (lower_pages, upper_pages) window closes, or abort encounter >> + * MAX_RECLAIM_RETRIES times of the following conditions: >> + * - No writeback-candidate memcgs found in a memcg tree walk. >> + * - Shrinking a writeback-candidate memcg failed. >> + * >> + * For shrink_worker(), it passes lower=thr and upper=zswap_total_pages(). >> + * The @upper limit is refreshed in each iteration by re-evaluating >> + * zswap_total_pages(), and the window closes once the total falls >> + * below the threshold. > > This is the wrong abstraction level, and it's obvious by the fact that > the function calls zswap_total_pages() again to recalcualte > 'upper_pages'. It gets much worse in the next patch as well. > > The lower_pages and upper_pages thing is also unnecessarily hard to > follow. > > The core of the reuse here is the retry logic. So maybe keep the memcg > iteration in the callers, and define a function that takes in one memcg > and reclaims one batch from it? failures and attempts can be passed into > the function to maintain the state across scans of different memcgs, > like zswap_shrink_walk_arg? > > WDYT? Perhaps something like this? struct zswap_shrink_state { int attempts; int failures; bool stop; }; static bool zswap_shrink_no_candidate(struct zswap_shrink_state *s) { if (!s->attempts && ++s->failures == MAX_RECLAIM_RETRIES) return true; s->attempts = 0; return false; } static long zswap_shrink_one(struct mem_cgroup *memcg, struct zswap_shrink_state *s) { long shrunk; shrunk = shrink_memcg(memcg, NR_ZSWAP_WB_BATCH); if (shrunk == -ENOENT) return 0; s->attempts++; if (shrunk <= 0 && ++s->failures == MAX_RECLAIM_RETRIES) s->stop = true; return shrunk; } static void shrink_worker(struct work_struct *w) { struct zswap_shrink_state s = {}; unsigned long thr; /* Reclaim down to the accept threshold */ thr = zswap_accept_thr_pages(); while (zswap_total_pages() > thr) { struct mem_cgroup *memcg; cond_resched(); memcg = zswap_iter_global(); if (!memcg) { if (zswap_shrink_no_candidate(&s)) break; continue; } zswap_shrink_one(memcg, &s); /* Drop the extra reference taken by the iterator. */ mem_cgroup_put(memcg); if (s.stop) break; } } We could also fold the logic of zswap_shrink_no_candidate() into zswap_shrink_one(), but adding a !memcg check inside zswap_shrink_one() feels a bit awkward. WDYT? Thanks, Hao