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

Matthew Wilcox
2020-03-25T15:32:34Z

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.