Re: Splitting a THP beyond EOF

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

On Tue, Oct 20, 2020 at 02:43:57AM +0100, Matthew Wilcox wrote:
> I think the easiest way to fix this is to decline to allocate readahead
> pages beyond EOF.  That is, if we have a file which is, say, 61 pages
> long, read the last 5 pages into an order-2 THP and an order-0 THP
> instead of allocating an order-3 THP and zeroing the last three pages.

Oh yeah, really easy.

+++ b/mm/readahead.c
@@ -481,6 +481,11 @@ void page_cache_ra_order(struct readahead_control *ractl,
                        if (order == 1)
                                order = 0;
                }
+               /* Don't allocate pages past EOF */
+               while (index + (1UL << order) - 1 > limit) {
+                       if (--order == 1)
+                               order = 0;
+               }
                err = ra_alloc_page(ractl, index, mark, order, gfp);
                if (err)
                        break;

I've added that to an earlier patch and I've pushed out commit
cd3fa4bc6516 as the head of
http://git.infradead.org/users/willy/pagecache.git/shortlog