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Received: from willy by casper.infradead.org with local (Exim 4.97.1 #2 (Red Hat Linux)) id 1sJBXO-000000027Ft-3j5X; Mon, 17 Jun 2024 12:32:43 +0000 Date: Mon, 17 Jun 2024 13:32:42 +0100 From: Matthew Wilcox To: "Pankaj Raghav (Samsung)" Cc: david@fromorbit.com, djwong@kernel.org, chandan.babu@oracle.com, brauner@kernel.org, akpm@linux-foundation.org, mcgrof@kernel.org, linux-mm@kvack.org, hare@suse.de, linux-kernel@vger.kernel.org, yang@os.amperecomputing.com, Zi Yan , linux-xfs@vger.kernel.org, p.raghav@samsung.com, linux-fsdevel@vger.kernel.org, hch@lst.de, gost.dev@samsung.com, cl@os.amperecomputing.com, john.g.garry@oracle.com Subject: Re: [PATCH v7 04/11] readahead: allocate folios with mapping_min_order in readahead Message-ID: References: <20240607145902.1137853-1-kernel@pankajraghav.com> <20240607145902.1137853-5-kernel@pankajraghav.com> <20240614092602.jc5qeoxy24xj6kl7@quentin> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240614092602.jc5qeoxy24xj6kl7@quentin> X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: BB4E01C001F X-Stat-Signature: bgz7arkqunzz83nuaan3t1159hzq9m6f X-Rspam-User: X-HE-Tag: 1718627569-755647 X-HE-Meta: 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 PUsAjktR hdczANFviwkxDVn18a3CacNk+ME7NKCuKnxqVXqYftycGXeCj4hJvVY+A2SHm59Xidm3GYg/1F/aGvn3/j4ejV5QgdXQcuyLZjZvjYxF3OMof4VVrzN+hqpz82Y+7mzv+Mo0vIEE9AAGnLzj71rZdDD7wenW+Htb0RiFVfqFvf3DdSMr3tnjn+HjFzT+NhMONdXBuE0SoO5+SOqrTZLJdU6PMUg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, 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 Fri, Jun 14, 2024 at 09:26:02AM +0000, Pankaj Raghav (Samsung) wrote: > > Hm, but we don't have a reference on this folio. So this isn't safe. > > That is why I added a check for mapping after read_pages(). You are > right, we can make it better. That's not enoughh. > > > + if (mapping != folio->mapping) > > > + nr_pages = min_nrpages; > > > + > > > + VM_BUG_ON_FOLIO(nr_pages < min_nrpages, folio); > > > + ractl->_index += nr_pages; > > > > Why not just: > > ractl->_index += min_nrpages; > > Then we will only move min_nrpages even if the folio we found had a > bigger order. Hannes patches (first patch) made sure we move the > ractl->index by folio_nr_pages instead of 1 and making this change will > defeat the purpose because without mapping order set, min_nrpages will > be 1. Hannes' patch is wrong. It's not safe to call folio_nr_pages() unless you have a reference to the folio. > @@ -266,10 +266,8 @@ void page_cache_ra_unbounded(struct readahead_control *ractl, > * alignment constraint in the page cache. > * > */ > - if (mapping != folio->mapping) > - nr_pages = min_nrpages; > + nr_pages = max(folio_nr_pages(folio), (long)min_nrpages); No. > Now we will still move respecting the min order constraint but if we had > a bigger folio and we do have a reference, then we move folio_nr_pages. You don't have a reference, so it's never safe.