From mboxrd@z Thu Jan  1 00:00:00 1970
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCHv6 08/37] filemap: handle huge pages in
 do_generic_file_read()
Date: Thu, 9 Feb 2017 13:55:05 -0800
Message-ID: <20170209215505.GW2267@bombadil.infradead.org>
References: <20170126115819.58875-1-kirill.shutemov@linux.intel.com>
 <20170126115819.58875-9-kirill.shutemov@linux.intel.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Cc: Theodore Ts'o <tytso@mit.edu>,
	Andreas Dilger <adilger.kernel@dilger.ca>, Jan Kara <jack@suse.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Hugh Dickins <hughd@google.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Ross Zwisler <ross.zwisler@linux.intel.com>,
	linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-block@vger.kernel.org
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Return-path: <owner-linux-mm@kvack.org>
Content-Disposition: inline
In-Reply-To: <20170126115819.58875-9-kirill.shutemov@linux.intel.com>
Sender: owner-linux-mm@kvack.org
List-Id: linux-ext4.vger.kernel.org

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?

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>