From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from zeniv.linux.org.uk ([195.92.253.2]:49302 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726911AbeINJbJ (ORCPT ); Fri, 14 Sep 2018 05:31:09 -0400 Date: Fri, 14 Sep 2018 05:18:32 +0100 From: Al Viro To: David Howells Cc: linux-fsdevel@vger.kernel.org, linux-afs@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 04/10] iov_iter: Add mapping and discard iterator types Message-ID: <20180914041831.GY19965@ZenIV.linux.org.uk> References: <153685389564.14766.11306559824641824935.stgit@warthog.procyon.org.uk> <153685392942.14766.3347355712333618914.stgit@warthog.procyon.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <153685392942.14766.3347355712333618914.stgit@warthog.procyon.org.uk> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Thu, Sep 13, 2018 at 04:52:09PM +0100, David Howells wrote: > Add two new iterator types to iov_iter: > > (1) ITER_MAPPING > > This walks through a set of pages attached to an address_space that > are pinned or locked, starting at a given page and offset and walking > for the specified amount of space. A facility to get a callback each > time a page is entirely processed is provided. > > This is useful for copying data from socket buffers to inodes in > network filesystems. Interesting... Questions: * what will hold those pages? IOW, where will you unlock/drop/whatnot those sucker? * "callback" sounds dangerous - it appears to imply that you won't copy to/from the same page twice. Not true for a lot of iov_iter users; what happens if you pass such a beast to them? * why not simply "build and populate ITER_BVEC aliasing a piece of mapping", possibly in "grab" and "grab+lock" variants? Those ITER_MAPPING do seem to be related to ITER_BVEC, at the very least. Note, BTW, that iov_iter_get_pages...() might mutate into something similar - "build and populate ITER_BVEC aliasing a piece of given iov_iter". Or, perhaps, a nicer-on-memory analogue of ITER_BVEC - with instead of as elements, with the same "populate from mapping" to get something similar to your functionality and "populate from iov_iter" for iov_iter_get_pages... replacement.