On Mon, Apr 20, 2020 at 1:43 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx/
>
> 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@xxxxxxxxxx>
Thanks,
Miklos
