On Fri, 2020-09-04 at 11:09 -0700, Andrew Morton wrote:
> 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?
Hi Andrew
Sorry, I am still not quite understanding your above three questions.
Based on my shallow understanding, ra_pages is associated with
read_ahead_kb. Seems ra_pages controls the maximum read-ahead window
size, but it doesn't work when the requested size exceeds ra_pages.
If I set the read_ahead_kb to 0, also, as Christoph mentioned, MTD
forcibly sets ra_pages to 0. I think the intention is that only wants
to disable read-ahead, however, doesn't want
generic_file_buffered_read() to split the request and read data with
4KB chunk size separately.
Thanks,
Bean
