Re: [PATCH v6 08/19] mm: Add readahead address space operation

Matthew Wilcox
2020-02-18T16:10:07Z

On Tue, Feb 18, 2020 at 05:21:47PM +1100, Dave Chinner wrote:
> On Mon, Feb 17, 2020 at 10:45:54AM -0800, Matthew Wilcox wrote:
> > From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> >
> > This replaces ->readpages with a saner interface:
> > - Return void instead of an ignored error code.
> > - Pages are already in the page cache when ->readahead is called.
>
> Might read better as:
>
> - Page cache is already populates with locked pages when
> ->readahead is called.

Will do.

> > - Implementation looks up the pages in the page cache instead of
> > having them passed in a linked list.
>
> Add:
>
> - cleanup of unused readahead handled by ->readahead caller, not
> the method implementation.

The readpages caller does that cleanup too, so it's not an advantage
to the readahead interface.

if (mapping->a_ops->readpages) {
ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
/* Clean up the remaining pages */
put_pages_list(pages);
goto out;
}

> > ``readpages``
> > called by the VM to read pages associated with the address_space
> > object. This is essentially just a vector version of readpage.
> > Instead of just one page, several pages are requested.
> > readpages is only used for read-ahead, so read errors are
> > ignored. If anything goes wrong, feel free to give up.
> > + This interface is deprecated; implement readahead instead.
>
> What is the removal schedule for the deprecated interface?

I mentioned that in the cover letter; once Dave Howells has the fscache
branch merged, I'll do the remaining filesystems. Should be within the
next couple of merge windows.

> > +/* The byte offset into the file of this readahead block */
> > +static inline loff_t readahead_offset(struct readahead_control *rac)
> > +{
> > + return (loff_t)rac->_start * PAGE_SIZE;
> > +}
>
> Urk. Didn't an early page use "offset" for the page index? That
> was was "mm: Remove 'page_offset' from readahead loop" did, right?
>
> That's just going to cause confusion to have different units for
> readahead "offsets"....

We are ... not consistent anywhere in the VM/VFS with our naming.
Unfortunately.

$ grep -n offset mm/filemap.c
391: * @start: offset in bytes where the range starts
...
815: pgoff_t offset = old->index;
...
2020: unsigned long offset; /* offset into pagecache page */
...
2257: *ppos = ((loff_t)index << PAGE_SHIFT) + offset;

That last one's my favourite. Not to mention the fine distinction you
and I discussed recently between offset_in_page() and page_offset().

Best of all, even our types encode the ambiguity of an 'offset'. We have
pgoff_t and loff_t (replacing the earlier off_t).

So, new rule. 'pos' is the number of bytes into a file. 'index' is the
number of PAGE_SIZE pages into a file. We don't use the word 'offset'
at all. 'length' as a byte count and 'count' as a page count seem like
fine names to me.

> > - if (aops->readpages) {
> > + if (aops->readahead) {
> > + aops->readahead(rac);
> > + readahead_for_each(rac, page) {
> > + unlock_page(page);
> > + put_page(page);
> > + }
>
> This needs a comment to explain the unwinding that needs to be done
> here. I'm not going to remember in a year's time that this is just
> for the pages that weren't submitted by ->readahead....

ACK.