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Thu, 21 Mar 2024 16:50:20 -0700 (PDT) MIME-Version: 1.0 References: <20240320020823.337644-1-yosryahmed@google.com> <20240320020823.337644-2-yosryahmed@google.com> In-Reply-To: From: Nhat Pham Date: Thu, 21 Mar 2024 16:50:07 -0700 Message-ID: Subject: Re: [PATCH 2/2] mm: zswap: remove nr_zswap_stored atomic To: Yosry Ahmed Cc: Andrew Morton , Johannes Weiner , Chengming Zhou , linux-mm@kvack.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: BA07EC0019 X-Stat-Signature: m495u7tc7iq3ug6bcnim3m7n5x94xijz X-Rspam-User: X-HE-Tag: 1711065021-564180 X-HE-Meta: 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 5iby8cI9 E28ozrQkXnZ7SL/xpBZHMulsyGTsaZFFy/O4nJHIjpzMoHmERVrzRN9LXyhdWCHMJlMcxtt36kD+8FPFe8EYF9dHxU7Bnu1Az61CD6MoK3d7lVDmJs9f5oYSXp/xO0iJvFqAYGLlupGRx8P/EGifVqpegkunwvSptfzcH0x7hpq8mjgKAzq3rPdwsX9nZjDswZkuQgBTSRt+ZCkxqmj/SrQ07ErnJtwna+q+KHG0kQW8Lxtq0jz8nxqyCIW9Kvfgsd0h9Ng8NMV+wJNZig0g9P3yhDlRSxRdwhXs2iwrG9xfts1DhmVtD5htpARQFbh1AH6/4QS2MsZrpUWXEQKvyInlzVIzEuDH0sQxQcjt2hKcfQbzd/EktXdVFxEgrGWXMzPhMHgzK5fcwP+lJL2HU8FnjHri8fez1dACxYjN8WawVqVVmWubdtS4BYg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.416737, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Mar 21, 2024 at 2:09=E2=80=AFPM Yosry Ahmed = wrote: > > On Tue, Mar 19, 2024 at 7:08=E2=80=AFPM Yosry Ahmed wrote: > > > > zswap_nr_stored is used to maintain the number of stored pages in zswap > > that are not same-filled pages. It is used in zswap_shrinker_count() to > > scale the number of freeable compressed pages by the compression ratio. > > That is, to reduce the amount of writeback from zswap with higher > > compression ratios as the ROI from IO diminishes. > > > > However, the need for this counter is questionable due to two reasons: > > - It is redundant. The value can be inferred from (zswap_stored_pages - > > zswap_same_filled_pages). Ah, I forgot about this. For context, nr_stored was originally a zswap_pool-specific stat, but I think Chengming has pulled it out and converted it into a global pool stat in an earlier patch - yet, globally, we already have zswap_stored_pages that is (mostly) the same counter. Might as well use existing counters (zswap_stored_pages) then, rather than a newly introduced counter. Probably will shave off a couple cycles here and there for the atomic increment/decrement :) > > - When memcgs are enabled, we use memcg_page_state(memcg, > > MEMCG_ZSWAPPED), which includes same-filled pages anyway (i.e. > > equivalent to zswap_stored_pages). This is fine I suppose. I was aware of this weird inaccuracy. However, for the CONFIG_MEMCG case, it was kinda silly to introduce the counter for per-cgroup same filled zswap pages, just for this one purpose, so I decided to accept the inaccuracy. > > > > Use zswap_stored_pages instead in zswap_shrinker_count() to keep things > > consistent whether memcgs are enabled or not, and add a comment about > > the number of freeable pages possibly being scaled down more than it > > should if we have lots of same-filled pages (i.e. inflated compression > > ratio). > > > > Remove nr_zswap_stored and one atomic operation in the store and free > > paths. > > > > Signed-off-by: Yosry Ahmed > > Any thoughts on this patch? Should I resend it separately? Might be worth resending it separately, but up to you and Andrew! Reviewed-by: Nhat Pham