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

Miklos Szeredi
2020-03-25T15:55:00Z

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