find_get_pages_contig

[Matthew Wilcox <willy@infradead.org>]

Hi Nick,

Sorry to trouble you about a patch from 2011, but even cregit can't find
a mailing list discussion to guide me.  The commit is
9cbb4cb21b19fff46cf1174d0ed699ef710e641c (mm: find_get_pages_contig fixlet)

I understand that checking mapping and index before taking the ref can
lead to false positives & negatives, but here's what the current code
looks like:

                head = compound_head(page);
                if (!page_cache_get_speculative(head))
                        goto retry;

                /* The page was split under us? */
                if (compound_head(page) != head)
                        goto put_page;

                /* Has the page moved? */
                if (unlikely(page != xas_reload(&xas)))
                        goto put_page;

                /*
                 * must check mapping and index after taking the ref.
                 * otherwise we can get both false positives and false
                 * negatives, which is just confusing to the caller.
                 */
                if (!page->mapping || page_to_pgoff(page) != xas.xa_index) {
                        put_page(page);
                        break;
                }

After checking that page is still at the right location (done by the
xas_reload() call up there), does checking mapping and page_to_pgoff
really check anything new?  It's my understanding that after I have a
ref on a page, it can't be moved within the mapping or to a new mapping.