Re: [PATCHv6 08/37] filemap: handle huge pages in do_generic_file_read()

From: Matthew Wilcox
Date: Thu Feb 09 2017 - 16:57:40 EST

On Thu, Jan 26, 2017 at 02:57:50PM +0300, Kirill A. Shutemov wrote:
> +++ b/mm/filemap.c
> @@ -1886,6 +1886,7 @@ static ssize_t do_generic_file_read(struct file *filp, loff_t *ppos,
> if (unlikely(page == NULL))
> goto no_cached_page;
> }
> + page = compound_head(page);

We got this page from find_get_page(), which gets it from
pagecache_get_page(), which gets it from find_get_entry() ... which
(unless I'm lost in your patch series) returns the head page. So this
line is redundant, right?

But then down in filemap_fault, we have:

VM_BUG_ON_PAGE(page->index != offset, page);

... again, maybe I'm lost somewhere in your patch series, but I don't see
anywhere you remove that line (or modify it). So are you not testing
with VM debugging enabled, or are you not doing a test which includes
mapping a file with huge pages, reading from it (to get the page in cache),
then faulting on an address that is not in the first 4kB of that 2MB?