From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sunil Mushran Subject: Re: [PATCH 1/2] fs: add SEEK_HOLE and SEEK_DATA flags Date: Fri, 22 Apr 2011 09:28:13 -0700 Message-ID: <4DB1AC9D.3010706@oracle.com> References: <1303414954-3315-1-git-send-email-josef@redhat.com> <20110422045054.GB17795@infradead.org> <20110422112852.GB1627@x4.trippels.de> <4DB16B72.1050702@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Cc: Markus Trippelsdorf , Christoph Hellwig , Josef Bacik , linux-kernel@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org To: Eric Blake Return-path: In-Reply-To: <4DB16B72.1050702@redhat.com> List-ID: On 04/22/2011 04:50 AM, Eric Blake wrote: > That blog also mentioned the useful idea of adding FIND_HOLE and > FIND_DATA, not implemented in Solaris, but which could easily be > provided as additional lseek constants in Linux to locate the start of > the next chunk without repositioning and which could ease application > programmer's life a bit. After all, cp wants to know where data ends > without repositioning (FIND_HOLE), read() that much data which > repositions in the process, then skip to the next chunk of data > (SEEK_DATA) - two lseek() calls per iteration if we have 4 constants, > but 3 per iteration if we only have SEEK_HOLE and have to manually rewind. while(1) { read(block); if (block_all_zeroes) lseek(SEEK_DATA); } What's wrong with the above? If this is the case, even SEEK_HOLE is not needed but should be added as it is already in Solaris. My problem with FIND_* is that we are messing with the well understood semantics of lseek(). And if generic_file_llseek_unlocked() treats SEEK_DATA as SEEK_CUR and SEEK_HOLE as SEEK_END (both with zero offset) then we don't even have to bother with the finding the "correct" error code.