Messages in this thread | | | Date | Mon, 20 Apr 2020 04:43:00 -0700 | From | Matthew Wilcox <> | Subject | Re: [PATCH v11 24/25] fuse: Convert from readpages to readahead |
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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.
> > + 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.
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