From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: A question on block_prepare_write() Date: Sat, 23 Oct 2010 10:40:36 -0700 Message-ID: <20101023104036.2bec05a0.akpm@linux-foundation.org> References: <1287841482.1681.36.camel@leonhard> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: linux-fsdevel , LKML To: Namhyung Kim Return-path: Received: from smtp1.linux-foundation.org ([140.211.169.13]:38927 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755406Ab0JWRku (ORCPT ); Sat, 23 Oct 2010 13:40:50 -0400 In-Reply-To: <1287841482.1681.36.camel@leonhard> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Sat, 23 Oct 2010 22:44:42 +0900 Namhyung Kim wrote: > Hello, > > I see block_prepare_write() has local variable wait[2] to keep track of > buffer_heads which are not up-to-date. But I'm wondering how it could be > guaranteed there will be no more than 2 such buffer_heads? Is there any > restriction on the usage of the function? Using MAX_BUF_PER_PAGE instead > of the magic number 2 is just useless? I couldn't find any comments or > documentation on this. > > Any of your comments would be greatly appreciated. TIA. :-) > block_prepare_write() may need to preread any buffer_head which are being only partially modified by the write(). Buffers which aren't being modified at all don't need to be preread. Buffers which are being fully modified don't need to be preread (because all their data is being overwritten). page: |-----------------------| buffer_heads: |-----|-----|-----|-----| area we're writing to: |---------| There can only be a maximum of two partially-modified buffers in the page.