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Received: from willy by casper.infradead.org with local (Exim 4.97.1 #2 (Red Hat Linux)) id 1sGfeu-00000009DiY-3Ghk; Mon, 10 Jun 2024 14:06:04 +0000 Date: Mon, 10 Jun 2024 15:06:04 +0100 From: Matthew Wilcox To: Usama Arif Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, david@redhat.com, ying.huang@intel.com, hughd@google.com, yosryahmed@google.com, nphamcs@gmail.com, chengming.zhou@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH v3 1/2] mm: store zero pages to be swapped out in a bitmap Message-ID: References: <20240610121820.328876-1-usamaarif642@gmail.com> <20240610121820.328876-2-usamaarif642@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Queue-Id: B571FA0003 X-Stat-Signature: bqdqciq7w1pd51bfadhqk7mwmzdfjm95 X-Rspam-User: X-Rspamd-Server: rspam11 X-HE-Tag: 1718028376-898587 X-HE-Meta: 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 Y8E6pl0Y OjPnxzP4y6UKhNSSRpbML8P0/2lk392JwzA2xmJD51f3aOmygeLzuwkQwJN4YyKom7NGDKwNaSlnJfT/bTPIkdOPHO9RH4Bn3v2Fc21aapKOXSem3tk1cQVOWEXcV/yrfenc9RtHKDzY2j9IxBEjzhwvNyQhhB9sU2728BiErMhz04Kd4oSEK4ON+rt6/DdAbB9ZH/oKkf8kPu1IayBYtZDGAMnPICMil7qP1 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000001, 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 Mon, Jun 10, 2024 at 02:56:09PM +0100, Usama Arif wrote: > I am guessing what you are suggesting is just do this? > >     if (is_folio_zero_filled(folio)) { >         swap_zeromap_folio_set(folio); >         folio_unlock(folio); >         return 0; >     } Right. > This is what I did initially while developing this, but when I started > looking into why zswap_store does  folio_start_writeback, folio_unlock, > folio_end_writeback I found: > > https://elixir.bootlin.com/linux/v6.9.3/source/Documentation/filesystems/locking.rst#L336 > > "If no I/O is submitted, the filesystem must run end_page_writeback() > against the page before returning from writepage." But that's advice to filesystem authors. File pages don't come this way; we only put anonyous pages into swap (except tmpfs). > If we have zswap enabled, the zero filled pages (infact any page that is > compressed), will be saved in zswap_entry and NR_WRITTEN will be wrongly > incremented. So the behaviour for NR_WRITTEN does not change in this patch > when encountering zero pages with zswap enabled (even if its wrong). We should fiz zswap too. > In order to fix NR_WRITTEN accounting for zswap, this patch series and any > other cases where no I/O is submitted but end_page_writeback is called > before returning to writepage, maybe we could add an argument to > __folio_end_writeback like below? There are a lot of calls to > folio_end_writeback and the behaviour of zeropage with regards to NR_WRITTEN > doesnt change with or without this patchseries with zswap enabled, so maybe > we could keep this independent of this series? I would be happy to submit > this as separate patch if it makes sense. It makes no sense at all.