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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 v10 24/25] fuse: Convert from readpages to readahead
Date: Wed, 25 Mar 2020 16:54:43 +0100
Message-ID: <CAJfpegtrhGamoSqD-3Svfj3-iTdAbfD8TP44H_o+HE+g+CAnCA@mail.gmail.com> (raw)
In-Reply-To: <20200325153228.GB22483@bombadil.infradead.org>

On Wed, Mar 25, 2020 at 4:32 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Mar 25, 2020 at 03:43:02PM +0100, Miklos Szeredi wrote:
> > >
> > > -       while ((page = readahead_page(rac))) {
> > > -               if (fuse_readpages_fill(&data, page) != 0)
> > > +               nr_pages = min(readahead_count(rac), fc->max_pages);
> >
> > Missing fc->max_read clamp.
>
> Yeah, I realised that.  I ended up doing ...
>
> +       unsigned int i, max_pages, nr_pages = 0;
> ...
> +       max_pages = min(fc->max_pages, fc->max_read / PAGE_SIZE);
>
> > > +               ia = fuse_io_alloc(NULL, nr_pages);
> > > +               if (!ia)
> > >                         return;
> > > +               ap = &ia->ap;
> > > +               __readahead_batch(rac, ap->pages, nr_pages);
> >
> > nr_pages = __readahead_batch(...)?
>
> That's the other bug ... this was designed for btrfs which has a fixed-size
> buffer.  But you want to dynamically allocate fuse_io_args(), so we need to
> figure out the number of pages beforehand, which is a little awkward.  I've
> settled on this for the moment:
>
>         for (;;) {
>                struct fuse_io_args *ia;
>                 struct fuse_args_pages *ap;
>
>                 nr_pages = readahead_count(rac) - nr_pages;
>                 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;
>                 __readahead_batch(rac, ap->pages, nr_pages);
>                 for (i = 0; i < nr_pages; i++) {
>                         fuse_wait_on_page_writeback(inode,
>                                                     readahead_index(rac) + i);
>                         ap->descs[i].length = PAGE_SIZE;
>                 }
>                 ap->num_pages = nr_pages;
>                 fuse_send_readpages(ia, rac->file);
>         }
>
> but I'm not entirely happy with that either.  Pondering better options.

I think that's fine.   Note how the original code possibly
over-allocates the the page array, because it doesn't know the batch
size beforehand.  So this is already better.

>
> > This will give consecutive pages, right?
>
> readpages() was already being called with consecutive pages.  Several
> filesystems had code to cope with the pages being non-consecutive, but
> that wasn't how the core code worked; if there was a discontiguity it
> would send off the pages that were consecutive and start a new batch.
>
> __readahead_batch() can't return fewer than nr_pages, so you don't need
> to check for that.

That's far from obvious.

I'd put a WARN_ON at least to make document the fact.

Thanks,
Miklos

  reply index

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-23 20:22 [PATCH v10 00/25] Change readahead API Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 01/25] mm: Move readahead prototypes from mm.h Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 02/25] mm: Return void from various readahead functions Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 03/25] mm: Ignore return value of ->readpages Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 04/25] mm: Move readahead nr_pages check into read_pages Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 05/25] mm: Add new readahead_control API Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 06/25] mm: Use readahead_control to pass arguments Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 07/25] mm: Rename various 'offset' parameters to 'index' Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 08/25] mm: rename readahead loop variable to 'i' Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 09/25] mm: Remove 'page_offset' from readahead loop Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 10/25] mm: Put readahead pages in cache earlier Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 11/25] mm: Add readahead address space operation Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 12/25] mm: Move end_index check out of readahead loop Matthew Wilcox
2020-03-23 20:49   ` Eric Biggers
2020-03-24  8:40   ` Christoph Hellwig
2020-03-23 20:22 ` [PATCH v10 13/25] mm: Add page_cache_readahead_unbounded Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 14/25] mm: Document why we don't set PageReadahead Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 15/25] mm: Use memalloc_nofs_save in readahead path Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 16/25] fs: Convert mpage_readpages to mpage_readahead Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 17/25] btrfs: Convert from readpages to readahead Matthew Wilcox
2020-03-26 16:53   ` David Sterba
2020-03-23 20:22 ` [PATCH v10 18/25] erofs: Convert uncompressed files " Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 19/25] erofs: Convert compressed " Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 20/25] ext4: Convert " Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 21/25] ext4: Pass the inode to ext4_mpage_readpages Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 22/25] f2fs: Convert from readpages to readahead Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 23/25] f2fs: Pass the inode to f2fs_mpage_readpages Matthew Wilcox
2020-03-23 20:22 ` [PATCH v10 24/25] fuse: Convert from readpages to readahead Matthew Wilcox
2020-03-25  9:42   ` Miklos Szeredi
2020-03-25 12:02     ` Matthew Wilcox
2020-03-25 14:43       ` Miklos Szeredi
2020-03-25 15:32         ` Matthew Wilcox
2020-03-25 15:54           ` Miklos Szeredi [this message]
2020-03-23 20:22 ` [PATCH v10 25/25] iomap: " Matthew Wilcox

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