Messages in this thread | | | From | Miklos Szeredi <> | Date | Mon, 20 Apr 2020 13:54:41 +0200 | Subject | Re: [PATCH v11 24/25] fuse: Convert from readpages to readahead |
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On Mon, Apr 20, 2020 at 1:43 PM Matthew Wilcox <willy@infradead.org> 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@mail.gmail.com/ > > 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@redhat.com>
Thanks, Miklos
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