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From: Al Viro <viro@ZenIV.linux.org.uk>
To: David Howells <dhowells@redhat.com>
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
Date: Fri, 14 Sep 2018 05:18:32 +0100	[thread overview]
Message-ID: <20180914041831.GY19965@ZenIV.linux.org.uk> (raw)
In-Reply-To: <153685392942.14766.3347355712333618914.stgit@warthog.procyon.org.uk>

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 <offset, bytes, pointer to pages array> instead of <offset, bytes, page> 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.

  reply	other threads:[~2018-09-14  4:18 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-13 15:51 [PATCH 00/10] iov_iter: Add new iters and use with AFS David Howells
2018-09-13 15:51 ` [PATCH 01/10] iov_iter: Separate type from direction and use accessor functions David Howells
2018-09-13 15:51 ` [PATCH 02/10] iov_iter: Renumber the ITER_* constants in uio.h David Howells
2018-09-13 15:52 ` [PATCH 03/10] iov_iter: Make count and iov_offset loff_t not size_t David Howells
2018-09-13 15:52 ` [PATCH 04/10] iov_iter: Add mapping and discard iterator types David Howells
2018-09-14  4:18   ` Al Viro [this message]
2018-09-14 12:57     ` Trond Myklebust
2018-09-17 21:32     ` David Howells
2018-09-17 20:58   ` David Howells
2018-09-13 15:52 ` [PATCH 05/10] afs: Better tracing of protocol errors David Howells
2018-09-13 15:52 ` [PATCH 06/10] afs: Set up the iov_iter before calling afs_extract_data() David Howells
2018-09-13 15:52 ` [PATCH 07/10] afs: Use ITER_MAPPING for writing David Howells
2018-09-13 15:52 ` [PATCH 08/10] afs: Add O_DIRECT read support David Howells
2018-09-13 15:52 ` [PATCH 09/10] afs: Add a couple of tracepoints to log I/O errors David Howells
2018-09-13 15:52 ` [PATCH 10/10] afs: Don't invoke the server to read data beyond EOF David Howells
2018-09-13 16:10 ` [PATCH 00/10] iov_iter: Add new iters and use with AFS Matthew Wilcox
2018-09-13 16:18 ` David Howells
2018-09-13 16:43   ` Matthew Wilcox
2018-09-13 17:05   ` David Howells
2018-09-13 17:58 ` Al Viro
  -- strict thread matches above, loose matches on Subject: below --
2018-08-06 13:16 David Howells
2018-08-06 13:17 ` [PATCH 04/10] iov_iter: Add mapping and discard iterator types David Howells

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