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Wed, 19 Feb 2020 13:47:03 +1100 (AEDT) Received: from hqpgpgate101.nvidia.com (Not Verified[216.228.121.13]) by hqnvemgate25.nvidia.com (using TLS: TLSv1.2, DES-CBC3-SHA) id ; Tue, 18 Feb 2020 18:46:27 -0800 Received: from hqmail.nvidia.com ([172.20.161.6]) by hqpgpgate101.nvidia.com (PGP Universal service); Tue, 18 Feb 2020 18:46:59 -0800 X-PGP-Universal: processed; by hqpgpgate101.nvidia.com on Tue, 18 Feb 2020 18:46:59 -0800 Received: from [10.110.48.28] (10.124.1.5) by HQMAIL107.nvidia.com (172.20.187.13) with Microsoft SMTP Server (TLS) id 15.0.1473.3; Wed, 19 Feb 2020 02:46:59 +0000 Subject: Re: [PATCH v6 09/19] mm: Add page_cache_readahead_limit To: Matthew Wilcox References: <20200217184613.19668-1-willy@infradead.org> <20200217184613.19668-16-willy@infradead.org> <1263603d-f446-c447-2eac-697d105fa76c@nvidia.com> <20200219022300.GJ24185@bombadil.infradead.org> X-Nvconfidentiality: public From: John Hubbard Message-ID: Date: Tue, 18 Feb 2020 18:46:59 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: <20200219022300.GJ24185@bombadil.infradead.org> X-Originating-IP: [10.124.1.5] X-ClientProxiedBy: HQMAIL105.nvidia.com (172.20.187.12) To HQMAIL107.nvidia.com (172.20.187.13) Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nvidia.com; s=n1; t=1582080388; bh=V/HjUqswaF1dKPtA+ar56SFisPTsJ76O2QClKAvYPMw=; h=X-PGP-Universal:Subject:To:CC:References:X-Nvconfidentiality:From: Message-ID:Date:User-Agent:MIME-Version:In-Reply-To: X-Originating-IP:X-ClientProxiedBy:Content-Type:Content-Language: Content-Transfer-Encoding; b=ekXkvC/QTudnr/E7HumrPKVft3Ce5bTjTn20IMsFFhe9t9mDZ0EgD+a2C09d+D85z AGPEc99roqIevW1cBB2LdQIY1R6pe8q72KUOxkuIp6p8ulVKsh7zjBwgatxBG8hoeY QCLpxGRClqbTVbQD9myhbXiWfGyh0XLZkFJPDeq8WH2Shs7nzWGWidWzrF8oIhNTD7 gsu9og2JxYod5aXUSdIlTtmakADanFstew6fv/i4LfFYqRVcDKJxsz2fwKiq29RqNZ JuJ7HKzL8sQILyEv0UsrM9YMdAbV3+bKN9dwYJItSxNZFn4ZwN8tC/8LQGXu1nSPCC H2F7kU+zXB7Eg== X-BeenThere: linux-erofs@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Development of Linux EROFS file system List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-xfs@vger.kernel.org, linux-kernel@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com, linux-mm@kvack.org, ocfs2-devel@oss.oracle.com, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-erofs@lists.ozlabs.org, linux-btrfs@vger.kernel.org Errors-To: linux-erofs-bounces+linux-erofs=archiver.kernel.org@lists.ozlabs.org Sender: "Linux-erofs" On 2/18/20 6:23 PM, Matthew Wilcox wrote: > On Tue, Feb 18, 2020 at 05:32:31PM -0800, John Hubbard wrote: >>> + page_cache_readahead_limit(inode->i_mapping, NULL, >>> + index, LONG_MAX, num_ra_pages, 0); >> >> >> LONG_MAX seems bold at first, but then again I can't think of anything smaller >> that makes any sense, and the previous code didn't have a limit either...OK. > > Probably worth looking at Dave's review of this and what we've just > negotiated on the other subthread ... LONG_MAX is gone. Great. OK, I see where it's going there. > >> I also wondered about the NULL file parameter, and wonder if we're stripping out >> information that is needed for authentication, given that that's what the newly >> written kerneldoc says the "file" arg is for. But it seems that if we're this >> deep in the fs code's read routines, file system authentication has long since >> been addressed. > > The authentication is for network filesystems. Local filesystems > generally don't use the 'file' parameter, and since we're going to be > calling back into the filesystem's own readahead routine, we know it's > not needed. > >> Any actually I don't yet (still working through the patches) see any authentication, >> so maybe that parameter will turn out to be unnecessary. >> >> Anyway, It's nice to see this factored out into a single routine. > > I'm kind of thinking about pushing the rac in the other direction too, > so page_cache_readahead_unlimited(rac, nr_to_read, lookahead_size). > >>> +/** >>> + * page_cache_readahead_limit - Start readahead beyond a file's i_size. >> >> >> Maybe: >> >> "Start readahead to a caller-specified end point" ? >> >> (It's only *potentially* beyond files's i_size.) > > My current tree has: > * page_cache_readahead_exceed - Start unchecked readahead. Sounds good. > > >>> + * @mapping: File address space. >>> + * @file: This instance of the open file; used for authentication. >>> + * @offset: First page index to read. >>> + * @end_index: The maximum page index to read. >>> + * @nr_to_read: The number of pages to read. >> >> >> How about: >> >> "The number of pages to read, as long as end_index is not exceeded." > > API change makes this irrelevant ;-) > >>> + * @lookahead_size: Where to start the next readahead. >> >> Pre-existing, but...it's hard to understand how a size is "where to start". >> Should we rename this arg? > > It should probably be lookahead_count. > >>> + * >>> + * This function is for filesystems to call when they want to start >>> + * readahead potentially beyond a file's stated i_size. If you want >>> + * to start readahead on a normal file, you probably want to call >>> + * page_cache_async_readahead() or page_cache_sync_readahead() instead. >>> + * >>> + * Context: File is referenced by caller. Mutexes may be held by caller. >>> + * May sleep, but will not reenter filesystem to reclaim memory. >> >> In fact, can we say "must not reenter filesystem"? > > I think it depends which side of the API you're looking at which wording > you prefer ;-) > > Yes. We should try to write these so that it's clear which way we're looking: in or out. thanks, -- John Hubbard NVIDIA