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From: Al Viro <viro@zeniv.linux.org.uk>
To: David Howells <dhowells@redhat.com>
Cc: Trond Myklebust <trondmy@hammerspace.com>,
	Anna Schumaker <anna.schumaker@netapp.com>,
	Steve French <sfrench@samba.org>,
	Matthew Wilcox <willy@infradead.org>,
	Jeff Layton <jlayton@redhat.com>,
	Dave Wysochanski <dwysocha@redhat.com>,
	linux-cachefs@redhat.com, linux-afs@lists.infradead.org,
	linux-nfs@vger.kernel.org, linux-cifs@vger.kernel.org,
	ceph-devel@vger.kernel.org, v9fs-developer@lists.sourceforge.net,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 01/32] iov_iter: Add ITER_MAPPING
Date: Sun, 19 Jul 2020 02:44:36 +0100	[thread overview]
Message-ID: <20200719014436.GG2786714@ZenIV.linux.org.uk> (raw)
In-Reply-To: <159465785214.1376674.6062549291411362531.stgit@warthog.procyon.org.uk>

On Mon, Jul 13, 2020 at 05:30:52PM +0100, David Howells wrote:
> Add an iterator, ITER_MAPPING, that walks through a set of pages attached
> to an address_space, starting at a given page and offset and walking for
> the specified amount of bytes.
> 
> The caller must guarantee that the pages are all present and they must be
> locked using PG_locked, PG_writeback or PG_fscache to prevent them from
> going away or being migrated whilst they're being accessed.
> 
> This is useful for copying data from socket buffers to inodes in network
> filesystems and for transferring data between those inodes and the cache
> using direct I/O.
> 
> Whilst it is true that ITER_BVEC could be used instead, that would require
> a bio_vec array to be allocated to refer to all the pages - which should be
> redundant if inode->i_pages also points to all these pages.
> 
> This could also be turned into an ITER_XARRAY, taking and xarray pointer
> instead of a mapping pointer.  It would be mostly trivial, except for the
> use of find_get_pages_contig() by iov_iter_get_pages*().
> 

My main problem here is that your iterate_mapping() assumes that STEP is
safe under rcu_read_lock(), with no visible mentioning of that fact.
Note, BTW, that iov_iter_for_each_range() quietly calls user-supplied
callback in such context.

Incidentally, do you ever have different steps for bvec and mapping?

> +	if (unlikely(iov_iter_is_mapping(i))) {
> +		/* We really don't want to fetch pages if we can avoid it */
> +		i->iov_offset += size;
> +		i->count -= size;
> +		return;

That's... not nice.  At the very least you want to cap size by i->count here
(and for discard case as well, while we are at it).

  reply	other threads:[~2020-07-19  1:45 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-13 16:30 [PATCH 00/32] fscache: Rewrite 2: Make the I/O interface use kiocb/iov_iter David Howells
2020-07-13 16:30 ` David Howells
2020-07-13 16:30 ` [PATCH 01/32] iov_iter: Add ITER_MAPPING David Howells
2020-07-13 16:30   ` David Howells
2020-07-19  1:44   ` Al Viro [this message]
2020-07-19  9:51   ` David Howells
2020-07-13 16:31 ` [PATCH 02/32] vm: Add wait/unlock functions for PG_fscache David Howells
2020-07-13 16:31   ` David Howells
2020-07-13 16:31 ` [PATCH 03/32] vfs: Export rw_verify_area() for use by cachefiles David Howells
2020-07-13 16:31 ` [PATCH 04/32] vfs: Provide S_CACHE_FILE inode flag David Howells
2020-07-13 16:31   ` David Howells
2020-07-13 16:31 ` [PATCH 05/32] mm: Provide lru_to_last_page() to get last of a page list David Howells
2020-07-13 16:31 ` [PATCH 06/32] cachefiles: Remove tree of active files and use S_CACHE_FILE inode flag David Howells
2020-07-13 16:31   ` David Howells
2020-07-13 16:31 ` [PATCH 07/32] fscache: Provide a simple thread pool for running ops asynchronously David Howells
2020-07-13 16:32 ` [PATCH 08/32] fscache: Replace the object management state machine David Howells
2020-07-13 16:32   ` David Howells
2020-07-13 16:32 ` [PATCH 09/32] fscache: Rewrite the I/O API based on iov_iter David Howells
2020-07-13 16:32 ` [PATCH 10/32] fscache: Remove fscache_wait_on_invalidate() David Howells
2020-07-13 16:32   ` David Howells
2020-07-13 16:32 ` [PATCH 11/32] fscache: Keep track of size of a file last set independently on the server David Howells
2020-07-13 16:32   ` David Howells
2020-07-13 16:32 ` [PATCH 12/32] fscache, cachefiles: Fix disabled histogram warnings David Howells
2020-07-13 16:32   ` David Howells
2020-07-13 16:33 ` [PATCH 13/32] fscache: Recast assertion in terms of cookie not being an index David Howells
2020-07-13 16:33 ` [PATCH 14/32] cachefiles: Remove some redundant checks on unsigned values David Howells
2020-07-13 16:33   ` David Howells
2020-07-13 16:33 ` [PATCH 15/32] cachefiles: trace: Log coherency checks David Howells
2020-07-13 16:33   ` David Howells
2020-07-13 16:33 ` [PATCH 16/32] cachefiles: Split cachefiles_drop_object() up a bit David Howells
2020-07-13 16:33   ` David Howells
2020-07-13 16:33 ` [PATCH 17/32] cachefiles: Implement new fscache I/O backend API David Howells
2020-07-13 16:33 ` [PATCH 18/32] cachefiles: Merge object->backer into object->dentry David Howells
2020-07-13 16:34 ` [PATCH 19/32] cachefiles: Implement a content-present indicator and bitmap David Howells
2020-07-13 16:34   ` David Howells
2020-07-13 16:34 ` [PATCH 20/32] cachefiles: Implement extent shaper David Howells
2020-07-13 16:34 ` [PATCH 21/32] cachefiles: Round the cachefile size up to DIO block size David Howells
2020-07-13 16:34 ` [PATCH 22/32] cachefiles: Implement read and write parts of new I/O API David Howells
2020-07-13 16:34   ` David Howells
2020-07-13 16:34 ` [PATCH 23/32] cachefiles: Add I/O tracepoints David Howells
2020-07-13 16:34   ` David Howells
2020-07-13 16:35 ` [PATCH 24/32] fscache: Add read helper David Howells
2020-07-13 16:35   ` David Howells
2020-07-13 16:35 ` [PATCH 25/32] fscache: Display cache-specific data in /proc/fs/fscache/objects David Howells
2020-07-13 16:35   ` David Howells
2020-07-13 16:35 ` [PATCH 26/32] fscache: Remove more obsolete stats David Howells
2020-07-13 16:35   ` David Howells
2020-07-13 16:35 ` [PATCH 27/32] fscache: New stats David Howells
2020-07-13 16:35 ` [PATCH 28/32] fscache, cachefiles: Rewrite invalidation David Howells
2020-07-13 16:35   ` David Howells
2020-07-13 16:36 ` [PATCH 29/32] fscache: Implement "will_modify" parameter on fscache_use_cookie() David Howells
2020-07-13 16:36 ` [PATCH 30/32] fscache: Provide resize operation David Howells
2020-07-13 16:36 ` [PATCH 31/32] fscache: Remove the update operation David Howells
2020-07-13 16:36 ` [PATCH 32/32] cachefiles: Shape write requests David Howells
2020-07-13 16:36   ` David Howells

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