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d=pankajraghav.com; s=MBO0001; t=1718640265; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=POREH1u3gVml+HC9orbYJuYAy8z1F4Z7+Xy+bewL5lE=; b=Y8EFvt7cE2WPLS33UV4Bnyzd2orjPU2Ro+Nc/DcZXQBmi5pP1ojEEdAWJY7tLuvUWr66eW SXSeu6J3nNanv5wjXt0mxXrv24ChuzM3iYekkImJ/fhLOs8q30HRfO4T+F4mp/wvRC+rzO vSEDxOXX1SoXK6FmHznnOwUVgZM1OEqFqdl9TEctSDYx3GE+zbkorHhi+eJPpPpa0zxCah t70wDX6kquhC1Z1K1oB65vGU7jXWXgq0YuuR5NFbshFdvxL80vFLTTNN/9W8aA1cmBKvtl /RC/IQ+Gh1a8p2WIJCQpz5mocGrneJcTJbTJQWmoZq3L2fXUI8vwADEJgmvmUg== Date: Mon, 17 Jun 2024 16:04:20 +0000 From: "Pankaj Raghav (Samsung)" To: Matthew Wilcox 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: <20240617160420.ifwlqsm5yth4g7eo@quentin> 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: X-Stat-Signature: oyi8cubi65fkz4qd3r3gbkms46s3qj9m X-Rspamd-Queue-Id: F34D940029 X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1718640269-21999 X-HE-Meta: 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 De+h2Cqj VusAPYrZMwB/gfGkHUTwY6aYAzB4UjqrEPsHDR3cL//lTx/2jdDb6TrlndKx+cSdkzWDJXMPLkHwvz1RO/7hspSRTppGjGLwPgJz0J6aT+STk/b9uBzcCDiXwZq2Z+W0EzctgL572xpsfAGXEe+ajoLSJFgm6w3Nh8gIapeGGzZtE+z92xdF2DD273Sh/uSsZsyOJw0ZN9w3YvzXufQTBXGl8dxJTIiP3jy9q7n5SwBc6nzFJG+bDtfJqxHf+vcqTQIUNCkljlEvQUeA= 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 Mon, Jun 17, 2024 at 01:32:42PM +0100, Matthew Wilcox wrote: > 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. I am hitting my head now because you have literally mentioned that in the comment: * next batch. This page may be the one we would * have intended to mark as Readahead, but we don't * have a stable reference to this page, and it's * not worth getting one just for that. I will move it by min_nrpages as follows: > ractl->_index += min_nrpages; So the following can still be there from Hannes patch as we have a stable reference: ractl->_workingset |= folio_test_workingset(folio); - ractl->_nr_pages++; + ractl->_nr_pages += folio_nr_pages(folio); + i += folio_nr_pages(folio); } Thanks for the clarification. -- Pankaj