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From: Miklos Szeredi <miklos@szeredi.hu>
To: Matthew Wilcox <willy@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, linux-btrfs@vger.kernel.org,
	linux-erofs@lists.ozlabs.org, linux-ext4@vger.kernel.org,
	linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com,
	ocfs2-devel@oss.oracle.com, linux-xfs <linux-xfs@vger.kernel.org>,
	Dave Chinner <dchinner@redhat.com>,
	William Kucharski <william.kucharski@oracle.com>
Subject: Re: [PATCH v11 24/25] fuse: Convert from readpages to readahead
Date: Mon, 20 Apr 2020 13:54:41 +0200
Message-ID: <CAJfpeguKAbE+_=ctxp+_3gtbqADevMPrRQ1XV6t8AHXbKwDKvg@mail.gmail.com> (raw)
In-Reply-To: <20200420114300.GB5820@bombadil.infradead.org>

On Mon, Apr 20, 2020 at 1:43 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Mon, Apr 20, 2020 at 01:14:17PM +0200, Miklos Szeredi wrote:
> > > +       for (;;) {
> > > +               struct fuse_io_args *ia;
> > > +               struct fuse_args_pages *ap;
> > > +
> > > +               nr_pages = readahead_count(rac) - nr_pages;
> >
> > Hmm.  I see what's going on here, but it's confusing.   Why is
> > __readahead_batch() decrementing the readahead count at the start,
> > rather than at the end?
> >
> > At the very least it needs a comment about why nr_pages is calculated this way.
>
> Because usually that's what we want.  See, for example, fs/mpage.c:
>
>         while ((page = readahead_page(rac))) {
>                 prefetchw(&page->flags);
>                 args.page = page;
>                 args.nr_pages = readahead_count(rac);
>                 args.bio = do_mpage_readpage(&args);
>                 put_page(page);
>         }
>
> fuse is different because it's trying to allocate for the next batch,
> not for the batch we're currently on.
>
> I'm a little annoyed because I posted almost this exact loop here:
>
> https://lore.kernel.org/linux-fsdevel/CAJfpegtrhGamoSqD-3Svfj3-iTdAbfD8TP44H_o+HE+g+CAnCA@mail.gmail.com/
>
> and you said "I think that's fine", modified only by your concern
> for it not being obvious that nr_pages couldn't be decremented by
> __readahead_batch(), so I modified the loop slightly to assign to
> nr_pages.  The part you're now complaining about is unchanged.

Your annoyance is perfectly understandable.   This is something I
noticed now, not back then.

>
> > > +               if (nr_pages > max_pages)
> > > +                       nr_pages = max_pages;
> > > +               if (nr_pages == 0)
> > > +                       break;
> > > +               ia = fuse_io_alloc(NULL, nr_pages);
> > > +               if (!ia)
> > > +                       return;
> > > +               ap = &ia->ap;
> > > +               nr_pages = __readahead_batch(rac, ap->pages, nr_pages);
> > > +               for (i = 0; i < nr_pages; i++) {
> > > +                       fuse_wait_on_page_writeback(inode,
> > > +                                                   readahead_index(rac) + i);
> >
> > What's wrong with ap->pages[i]->index?  Are we trying to wean off using ->index?
>
> It saves reading from a cacheline?  I wouldn't be surprised if the
> compiler hoisted the read from rac->_index to outside the loop and just
> iterated from rac->_index to rac->_index + nr_pages.

Hah, if such optimizations were worth anything with codepaths
involving roundtrips to userspace...

Anyway, I'll let these be, and maybe clean them up later.

Acked-by:  Miklos Szeredi <mszeredi@redhat.com>

Thanks,
Miklos

  reply index

Thread overview: 47+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-14 15:02 [PATCH v11 00/25] Change readahead API Matthew Wilcox
2020-04-14 15:02 ` [PATCH v11 01/25] mm: Move readahead prototypes from mm.h Matthew Wilcox
2020-04-15  9:10   ` Johannes Thumshirn
2020-04-14 15:02 ` [PATCH v11 02/25] mm: Return void from various readahead functions Matthew Wilcox
2020-04-14 15:02 ` [PATCH v11 03/25] mm: Ignore return value of ->readpages Matthew Wilcox
2020-04-15  9:17   ` Johannes Thumshirn
2020-04-14 15:02 ` [PATCH v11 04/25] mm: Move readahead nr_pages check into read_pages Matthew Wilcox
2020-04-15  9:19   ` Johannes Thumshirn
2020-04-14 15:02 ` [PATCH v11 05/25] mm: Add new readahead_control API Matthew Wilcox
2020-04-15  1:17   ` Andrew Morton
2020-04-15  2:18     ` Matthew Wilcox
2020-04-15  4:56       ` Andrew Morton
2020-04-15 11:22         ` Matthew Wilcox
2020-04-14 15:02 ` [PATCH v11 06/25] mm: Use readahead_control to pass arguments Matthew Wilcox
2020-04-15  9:30   ` Johannes Thumshirn
2020-04-14 15:02 ` [PATCH v11 07/25] mm: Rename various 'offset' parameters to 'index' Matthew Wilcox
2020-04-14 15:02 ` [PATCH v11 08/25] mm: rename readahead loop variable to 'i' Matthew Wilcox
2020-04-15  9:31   ` Johannes Thumshirn
2020-04-14 15:02 ` [PATCH v11 09/25] mm: Remove 'page_offset' from readahead loop Matthew Wilcox
2020-04-14 15:02 ` [PATCH v11 10/25] mm: Put readahead pages in cache earlier Matthew Wilcox
2020-04-14 15:02 ` [PATCH v11 11/25] mm: Add readahead address space operation Matthew Wilcox
2020-04-14 15:02 ` [PATCH v11 12/25] mm: Move end_index check out of readahead loop Matthew Wilcox
2020-04-14 15:02 ` [PATCH v11 13/25] mm: Add page_cache_readahead_unbounded Matthew Wilcox
2020-04-14 15:02 ` [PATCH v11 14/25] mm: Document why we don't set PageReadahead Matthew Wilcox
2020-04-14 15:02 ` [PATCH v11 15/25] mm: Use memalloc_nofs_save in readahead path Matthew Wilcox
2020-04-14 15:02 ` [PATCH v11 16/25] fs: Convert mpage_readpages to mpage_readahead Matthew Wilcox
2020-06-16 22:36   ` [Cluster-devel] " Andreas Gruenbacher
2020-06-17  0:32     ` Matthew Wilcox
2020-06-17  0:57       ` Andreas Grünbacher
2020-06-17  2:21         ` Matthew Wilcox
2020-06-18 12:46           ` Andreas Gruenbacher
2020-06-18 15:03             ` Matthew Wilcox
2020-06-18 16:40               ` Andreas Gruenbacher
2020-04-14 15:02 ` [PATCH v11 17/25] btrfs: Convert from readpages to readahead Matthew Wilcox
2020-04-14 15:02 ` [PATCH v11 18/25] erofs: Convert uncompressed files " Matthew Wilcox
2020-04-14 15:02 ` [PATCH v11 19/25] erofs: Convert compressed " Matthew Wilcox
2020-04-21  5:42   ` Andrew Morton
2020-04-21  7:28     ` Gao Xiang
2020-04-14 15:02 ` [PATCH v11 20/25] ext4: Convert " Matthew Wilcox
2020-04-14 15:02 ` [PATCH v11 21/25] ext4: Pass the inode to ext4_mpage_readpages Matthew Wilcox
2020-04-14 15:02 ` [PATCH v11 22/25] f2fs: Convert from readpages to readahead Matthew Wilcox
2020-04-14 15:02 ` [PATCH v11 23/25] f2fs: Pass the inode to f2fs_mpage_readpages Matthew Wilcox
2020-04-14 15:02 ` [PATCH v11 24/25] fuse: Convert from readpages to readahead Matthew Wilcox
2020-04-20 11:14   ` Miklos Szeredi
2020-04-20 11:43     ` Matthew Wilcox
2020-04-20 11:54       ` Miklos Szeredi [this message]
2020-04-14 15:02 ` [PATCH v11 25/25] iomap: " Matthew Wilcox

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