mm: stop filemap_read() from grabbing a superfluous page

Under some circumstances, filemap_read() will allocate sufficient pages
to read to the end of the file, call readahead/readpages on them and
copy the data over - and then it will allocate another page at the EOF
and call readpage on that and then ignore it.  This is unnecessary and a
waste of time and resources.

filemap_read() *does* check for this, but only after it has already done
the allocation and I/O.  Fix this by checking before calling
filemap_get_pages() also.

Link: https://lkml.kernel.org/r/163472463105.3126792.7056099385135786492.stgit@warthog.procyon.org.uk
Link: https://lore.kernel.org/r/160588481358.3465195.16552616179674485179.stgit@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/163456863216.2614702.6384850026368833133.stgit@warthog.procyon.org.uk/Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -2625,6 +2625,9 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
 		if ((iocb->ki_flags & IOCB_WAITQ) && already_read)
 			iocb->ki_flags |= IOCB_NOWAIT;
 
+		if (unlikely(iocb->ki_pos >= i_size_read(inode)))
+			break;
+
 		error = filemap_get_pages(iocb, iter, &pvec);
 		if (error < 0)
 			break;