From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io0-f195.google.com ([209.85.223.195]:34363 "EHLO mail-io0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751276AbdH2RoH (ORCPT ); Tue, 29 Aug 2017 13:44:07 -0400 Received: by mail-io0-f195.google.com with SMTP id d81so3761397ioj.1 for ; Tue, 29 Aug 2017 10:44:07 -0700 (PDT) Subject: Re: Decompression success/failure dependent on PAGE_SIZE? To: =?UTF-8?Q?Marek_Beh=c3=ban?= , linux-btrfs@vger.kernel.org References: <20170829184342.2f6ec0b0@dellmb.labs.office.nic.cz> From: Nikolay Borisov Message-ID: Date: Tue, 29 Aug 2017 20:44:03 +0300 MIME-Version: 1.0 In-Reply-To: <20170829184342.2f6ec0b0@dellmb.labs.office.nic.cz> Content-Type: text/plain; charset=utf-8 Sender: linux-btrfs-owner@vger.kernel.org List-ID: On 29.08.2017 19:43, Marek Behún wrote: > Hello, > > so I've been studying the linux btrfs code and have come across this: > > in inode.c function uncompress_inline the max_size size variable is set > to min(max_size, PAGE_SIZE) and only max_size of output data are > decompressed. > > The code for compression (in lzo.c for example) uses PAGE_SIZEd chunks > to compress an inline extent. > > If I understand it correctly, then if the filesystem is created and used > on a computer with PAGE_SIZE for example 16KB, and an extent of size > 16KB is compressed to (for example 9KB) and stored as inline extent, > and then the filesystem is mounted on a computer with PAGE_SIZE = 4KB, > reading the extent will result in a failure or incomplete read. > > Is this a bug, or is this behaviour a feature? Please have a look in btrfs_check_super_valid(), which is being called from open_ctree, which in turn is called when a filesystem is mounted. Currently btrfs supports a single blocksize value - PAGE_SIZE, also it's forbidden to mount a filesystem on systems with different page_sizes. When the sub-page blocksizes patch is merged this might be have to be taken into consideration but certainly not at present times. > > Thank you. > > Marek > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html >