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From: David Howells <dhowells@redhat.com>
To: Christoph Hellwig <hch@lst.de>
Cc: dhowells@redhat.com, Trond Myklebust <trondmy@hammerspace.com>,
	Anna Schumaker <anna.schumaker@netapp.com>,
	Steve French <sfrench@samba.org>,
	Dominique Martinet <asmadeus@codewreck.org>,
	Jeff Layton <jlayton@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,
	David Wysochanski <dwysocha@redhat.com>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 14/33] fscache, cachefiles: Add alternate API to use kiocb for read/write to cache
Date: Tue, 16 Feb 2021 15:08:06 +0000	[thread overview]
Message-ID: <1444259.1613488086@warthog.procyon.org.uk> (raw)
In-Reply-To: <20210216104914.GA28196@lst.de>

Christoph Hellwig <hch@lst.de> wrote:

> > Filesystems wanting to use the new API must #define FSCACHE_USE_NEW_IO_API
> > before #including the header
> 
> What exactly does this ifdef buys us?  It seems like the old and new
> APIs don't even conflict.

I was asked to add this.  The APIs look like they don't conflict, but you
can't mix them for a given file because of the differing behaviour of the
PG_fscache flag.  It also makes it much easier to make sure you don't miss
something.  That has happened and it led to some strange effects before we
worked out what was going on.

> And if we really need an ifdef I'd rather use that for the old code to make
> grepping for that easier.

I can do it that way - but this doesn't require changing filesystems that
aren't being changed.  The intent would be to eliminate the #ifdef in a cycle
or two anyway.

Besides, there are 5 filesystems that use this, and two of them are converted
here.  grep would only return three hits: one each in fs/9p/cache.h,
fs/cifs/fscache.h and fs/nfs/fscache.h.

OTOH, I suppose it might dissuade people from adding new usages of the old
API.

> > +	if (ki->term_func) {
> > +		if (ret < 0)
> > +			ki->term_func(ki->term_func_priv, ret);
> > +		else
> > +			ki->term_func(ki->term_func_priv, ki->skipped + ret);
> 
> Why not simplify:
> 
> 		if (ret > 0)
> 			ret += ki->skipped;
> 		ki->term_func(ki->term_func_priv, ret);

Could do that I suppose.  The optimiser will make them the same anyway.

I still wonder if I should do something with ret2 as obtained from the kiocb
completion function:

  static void cachefiles_read_complete(struct kiocb *iocb, long ret, long ret2)

Can we consolidate to one return value?

> > +	/* If the caller asked us to seek for data before doing the read, then
> > +	 * we should do that now.  If we find a gap, we fill it with zeros.
> > +	 */
> 
> FYI, this is not the normal kernel comment style..

I've been following the network style.

> > +	ret = rw_verify_area(READ, file, &ki->iocb.ki_pos, len - skipped);
> > +	if (ret < 0)
> > +		goto presubmission_error_free;
> > +
> > +	get_file(ki->iocb.ki_filp);
> > +
> > +	old_nofs = memalloc_nofs_save();
> > +	ret = call_read_iter(file, &ki->iocb, iter);
> > +	memalloc_nofs_restore(old_nofs);
> 
> As mentioned before I think all this magic belongs in to a helper
> in the VFS.

You suggested vfs_iocb_iter_read() in your reply to another patch, but it
occurs to me that that doesn't have memalloc_nofs_*() in it.  I could hoist
the memalloc_nofs stuff out and use those helpers.

> > +static const struct netfs_cache_ops cachefiles_netfs_cache_ops = {
> > +	.end_operation		= cachefiles_end_operation,
> > +	.read			= cachefiles_read,
> > +	.write			= cachefiles_write,
> > +	.expand_readahead	= NULL,
> > +	.prepare_read		= cachefiles_prepare_read,
> > +};
> ...
> Also at least in linux-next ->read and ->write seem to never actually
> be called.

See netfs_read_from_cache() and netfs_rreq_do_write_to_cache() in
fs/netfs/read_helper.c.  Look for "cres->ops->".

> > +{
> > +	struct cachefiles_object *object;
> > +	struct cachefiles_cache *cache;
> > +	struct path path;
> > +	struct file *file;
> > +
> > +	_enter("");
> > +
> > +	object = container_of(op->op.object,
> > +			      struct cachefiles_object, fscache);
> > +	cache = container_of(object->fscache.cache,
> > +			     struct cachefiles_cache, cache);
> > +
> > +	path.mnt = cache->mnt;
> > +	path.dentry = object->backer;
> > +	file = open_with_fake_path(&path, O_RDWR | O_LARGEFILE | O_DIRECT,
> > +				   d_inode(object->backer), cache->cache_cred);
> 
> I think this should be plain old dentry_open?

open_with_fake_path() sets FMODE_NOACCOUNT.  In the fscache-iter branch, the
file is held open a lot longer and then ENFILE/EMFILE starts being a serious
problem.

That said, I'm considering changing things so that all the data in the cache
is held in one or a few files with an index to locate things - at which point
this issue goes away.

> > +	op = fscache_alloc_retrieval(cookie, NULL, NULL, NULL);
> > +	if (!op)
> > +		return -ENOMEM;
> > +	//atomic_set(&op->n_pages, 1);
> 
> ???

I should remove that - it kind of got left behind.  That was necessary for the
old API, but a whole load of this code, including the fscache_retrieval struct
will be going away when the cookie and operation handling get rewritten.

> > +{
> > +	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
> > +		return __fscache_begin_read_operation(rreq, cookie);
> > +	else
> > +		return -ENOBUFS;
> > +}
> 
> No need for an else after a return.  I personally also prefer to always
> handle the error case first, ala:

It's not precisely an error case, more a "fallback required" case.

>         if (!fscache_cookie_valid(cookie) || !fscache_cookie_enabled(cookie))
> 	                return -ENOBUFS;
> 	return __fscache_begin_read_operation(rreq, cookie);
> 
> Also do we really need this inline fast path to start with?

Yes.  fscache might be compiled out, in which case we'll never go down the
slow path.  And the common case is that cookie == NULL, so let's not jump out
of line if we don't have to.

David


  parent reply	other threads:[~2021-02-16 15:09 UTC|newest]

Thread overview: 81+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-15 15:44 [PATCH 00/33] Network fs helper library & fscache kiocb API [ver #3] David Howells
2021-02-15 15:44 ` [PATCH 01/33] iov_iter: Add ITER_XARRAY David Howells
2021-02-15 15:44 ` [PATCH 02/33] mm: Add an unlock function for PG_private_2/PG_fscache David Howells
2021-02-16 10:26   ` Christoph Hellwig
2021-02-15 15:44 ` [PATCH 03/33] mm: Implement readahead_control pageset expansion David Howells
2021-02-16 10:32   ` Christoph Hellwig
2021-02-16 13:22     ` Matthew Wilcox
2021-02-17 14:36       ` Mike Marshall
2021-02-17 14:36         ` Mike Marshall
2021-02-17 15:42       ` David Howells
2021-02-17 16:59         ` Mike Marshall
2021-02-17 16:59           ` Mike Marshall
2021-02-17 22:20         ` David Howells
2021-02-16 11:48   ` David Howells
2021-02-17 16:13   ` Matthew Wilcox
2021-02-17 22:34   ` David Howells
2021-02-17 22:49     ` Matthew Wilcox
2021-02-18 17:47   ` David Howells
2021-02-15 15:45 ` [PATCH 04/33] vfs: Export rw_verify_area() for use by cachefiles David Howells
2021-02-16 10:26   ` Christoph Hellwig
2021-02-16 11:55   ` David Howells
2021-02-15 15:45 ` [PATCH 05/33] netfs: Make a netfs helper module David Howells
2021-02-15 15:45 ` [PATCH 06/33] netfs, mm: Move PG_fscache helper funcs to linux/netfs.h David Howells
2021-02-15 15:45 ` [PATCH 07/33] netfs, mm: Add unlock_page_fscache() and wait_on_page_fscache() David Howells
2021-02-15 15:45 ` [PATCH 08/33] netfs: Provide readahead and readpage netfs helpers David Howells
2021-02-15 15:45 ` [PATCH 09/33] netfs: Add tracepoints David Howells
2021-02-15 15:46 ` [PATCH 10/33] netfs: Gather stats David Howells
2021-02-15 15:46 ` [PATCH 11/33] netfs: Add write_begin helper David Howells
2021-02-15 15:46 ` [PATCH 12/33] netfs: Define an interface to talk to a cache David Howells
2021-02-15 15:46 ` [PATCH 13/33] netfs: Hold a ref on a page when PG_private_2 is set David Howells
2021-02-15 15:47 ` [PATCH 14/33] fscache, cachefiles: Add alternate API to use kiocb for read/write to cache David Howells
2021-02-16 10:49   ` Christoph Hellwig
2021-02-16 15:08   ` David Howells [this message]
2021-02-15 15:47 ` [PATCH 15/33] afs: Disable use of the fscache I/O routines David Howells
2021-02-15 15:47 ` [PATCH 16/33] afs: Pass page into dirty region helpers to provide THP size David Howells
2021-02-15 15:47 ` [PATCH 17/33] afs: Print the operation debug_id when logging an unexpected data version David Howells
2021-02-15 15:47 ` [PATCH 18/33] afs: Move key to afs_read struct David Howells
2021-02-15 15:47 ` [PATCH 19/33] afs: Don't truncate iter during data fetch David Howells
2021-02-15 15:48 ` [PATCH 20/33] afs: Log remote unmarshalling errors David Howells
2021-02-15 15:48 ` [PATCH 21/33] afs: Set up the iov_iter before calling afs_extract_data() David Howells
2021-02-15 15:48 ` [PATCH 22/33] afs: Use ITER_XARRAY for writing David Howells
2021-02-15 15:48 ` [PATCH 23/33] afs: Wait on PG_fscache before modifying/releasing a page David Howells
2021-02-15 15:49 ` [PATCH 24/33] afs: Extract writeback extension into its own function David Howells
2021-02-15 15:49 ` [PATCH 25/33] afs: Prepare for use of THPs David Howells
2021-02-15 15:49 ` [PATCH 26/33] afs: Use the fs operation ops to handle FetchData completion David Howells
2021-02-15 15:49 ` [PATCH 27/33] afs: Use new fscache read helper API David Howells
2021-02-15 15:49 ` [PATCH 28/33] ceph: disable old fscache readpage handling David Howells
2021-02-15 15:50 ` [PATCH 29/33] ceph: rework PageFsCache handling David Howells
2021-02-15 15:50 ` [PATCH 30/33] ceph: fix fscache invalidation David Howells
2021-02-15 15:50 ` [PATCH 31/33] ceph: convert readpage to fscache read helper David Howells
2021-02-15 15:50 ` [PATCH 32/33] ceph: plug write_begin into " David Howells
2021-02-15 15:51 ` [PATCH 33/33] ceph: convert ceph_readpages to ceph_readahead David Howells
2021-02-15 18:05 ` [PATCH 00/33] Network fs helper library & fscache kiocb API [ver #3] Jeff Layton
2021-02-15 18:05   ` Jeff Layton
2021-02-16  0:40   ` Steve French
2021-02-16  0:40     ` Steve French
2021-02-16  2:10     ` Matthew Wilcox
2021-02-16  5:18       ` Steve French
2021-02-16  5:18         ` Steve French
2021-02-16  5:22       ` Steve French
2021-02-16  5:22         ` Steve French
2021-02-23 20:27         ` Matthew Wilcox
2021-02-23 20:27           ` [f2fs-dev] " Matthew Wilcox
2021-02-24  4:57           ` Steve French
2021-02-24  4:57             ` [f2fs-dev] " Steve French
2021-02-24  4:57             ` Steve French
2021-02-24 13:32       ` David Howells
2021-02-24 15:51         ` Matthew Wilcox
2021-02-16 11:01     ` Jeff Layton
2021-02-16 11:01       ` Jeff Layton
2021-02-15 22:46 ` [PATCH 34/33] netfs: Use in_interrupt() not in_softirq() David Howells
2021-02-16  8:42   ` Christoph Hellwig
2021-02-16  9:06     ` Sebastian Andrzej Siewior
2021-02-16  9:29   ` David Howells
2021-02-16  9:30     ` Christoph Hellwig
2021-02-18 14:02     ` [PATCH 34/33] netfs: Pass flag rather than use in_softirq() David Howells
2021-02-18 15:06       ` Marc Dionne
2021-02-18 15:06         ` Marc Dionne
2021-02-18 15:16       ` Marc Dionne
2021-02-18 15:16         ` Marc Dionne
2021-02-19  9:01       ` Sebastian Andrzej Siewior

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