linux-ext4 - Re: [PATCH v10 24/25] fuse: Convert from readpages to readahead

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Date:   Wed, 25 Mar 2020 08:32:28 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-erofs@...ts.ozlabs.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
        ocfs2-devel@....oracle.com, linux-xfs <linux-xfs@...r.kernel.org>,
        Dave Chinner <dchinner@...hat.com>,
        William Kucharski <william.kucharski@...cle.com>
Subject: Re: [PATCH v10 24/25] fuse: Convert from readpages to readahead

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.

> 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.