Re: [PATCH v6 07/19] mm: Put readahead pages in cache earlier

Matthew Wilcox
2020-02-19T14:41:18Z

On Tue, Feb 18, 2020 at 04:01:43PM -0800, John Hubbard wrote:
> How about this instead? It uses the "for" loop fully and more naturally,
> and is easier to read. And it does the same thing:
>
> static inline struct page *readahead_page(struct readahead_control *rac)
> {
> struct page *page;
>
> if (!rac->_nr_pages)
> return NULL;
>
> page = xa_load(&rac->mapping->i_pages, rac->_start);
> VM_BUG_ON_PAGE(!PageLocked(page), page);
> rac->_batch_count = hpage_nr_pages(page);
>
> return page;
> }
>
> static inline struct page *readahead_next(struct readahead_control *rac)
> {
> rac->_nr_pages -= rac->_batch_count;
> rac->_start += rac->_batch_count;
>
> return readahead_page(rac);
> }
>
> #define readahead_for_each(rac, page) \
> for (page = readahead_page(rac); page != NULL; \
> page = readahead_page(rac))

I'll go you one better ... how about we do this instead:

static inline struct page *readahead_page(struct readahead_control *rac)
{
struct page *page;

BUG_ON(rac->_batch_count > rac->_nr_pages);
rac->_nr_pages -= rac->_batch_count;
rac->_index += rac->_batch_count;
rac->_batch_count = 0;

if (!rac->_nr_pages)
return NULL;

page = xa_load(&rac->mapping->i_pages, rac->_index);
VM_BUG_ON_PAGE(!PageLocked(page), page);
rac->_batch_count = hpage_nr_pages(page);

return page;
}

#define readahead_for_each(rac, page) \
while ((page = readahead_page(rac)))

No more readahead_next() to forget to add to filesystems which don't use
the readahead_for_each() iterator. Ahem.