linux-ext4 - Re: [PATCHv6 08/37] filemap: handle huge pages in do_generic_file

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Date:   Thu, 9 Feb 2017 13:55:05 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jan Kara <jack@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-block@...r.kernel.org
Subject: Re: [PATCHv6 08/37] filemap: handle huge pages in
 do_generic_file_read()

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?