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

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Date:   Mon, 13 Feb 2017 18:33:42 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        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, 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