On Fri, 4 Sep 2020 16:48:07 +0200 Bean Huo <[email protected]> wrote:
> From: Bean Huo <[email protected]> > > Current generic_file_buffered_read() will break up the larger batches of pages > and read data in single page length in case of ra->ra_pages == 0. This patch > is > to allow it to pass the batches of pages down to the device if the supported > maximum IO size >= the requested size. > > ... > > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -2062,6 +2062,7 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, > struct file *filp = iocb->ki_filp; > struct address_space *mapping = filp->f_mapping; > struct inode *inode = mapping->host; > + struct backing_dev_info *bdi = inode_to_bdi(mapping->host); > struct file_ra_state *ra = &filp->f_ra; > loff_t *ppos = &iocb->ki_pos; > pgoff_t index; > @@ -2098,9 +2099,14 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, > if (!page) { > if (iocb->ki_flags & IOCB_NOIO) > goto would_block; > - page_cache_sync_readahead(mapping, > - ra, filp, > - index, last_index - index); > + > + if (!ra->ra_pages && bdi->io_pages >= last_index - > index) > + __do_page_cache_readahead(mapping, filp, index, > + last_index - index, > 0); > + else > + page_cache_sync_readahead(mapping, ra, filp, > + index, > + last_index - index); > page = find_get_page(mapping, index); > if (unlikely(page == NULL)) > goto no_cached_page; I assume this is a performance patch. What are the observed changes in behaviour? What is special about ->ra_pages==0? Wouldn't this optimization still be valid if ->ra_pages==2? Doesn't this defeat the purpose of having ->ra_pages==0?
