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

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Matthew Wilcox <willy@infradead.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	"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
Subject: Re: [PATCHv6 08/37] filemap: handle huge pages in do_generic_file_read()
Date: Mon, 13 Feb 2017 18:33:42 +0300	[thread overview]
Message-ID: <20170213153342.GE20394@node.shutemov.name> (raw)
In-Reply-To: <20170209215505.GW2267@bombadil.infradead.org>

On Thu, Feb 09, 2017 at 01:55:05PM -0800, Matthew Wilcox wrote:
> 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?

No. pagecache_get_page() returns subpage. See description of the first
patch.

> 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).

This should be fine as find_get_page() returns subpage.

> 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?
> 
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-- 
 Kirill A. Shutemov