linux-kernel - Re: [PATCHv4 12/39] thp, mm: rewrite add_to_page_cache

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Date:	Thu, 23 May 2013 09:00:01 -0700
From:	Dave Hansen <dave@...1.net>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC:	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Hillf Danton <dhillf@...il.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 12/39] thp, mm: rewrite add_to_page_cache_locked() to
 support huge pages

On 05/23/2013 07:36 AM, Kirill A. Shutemov wrote:
>>> +	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE_PAGECACHE)) {
>>> +		BUILD_BUG_ON(HPAGE_CACHE_NR > RADIX_TREE_PRELOAD_NR);
>>> +		nr = hpage_nr_pages(page);
>>> +	} else {
>>> +		BUG_ON(PageTransHuge(page));
>>> +		nr = 1;
>>> +	}
>>
>> Why can't this just be
>>
>> 		nr = hpage_nr_pages(page);
>>
>> Are you trying to optimize for the THP=y, but THP-pagecache=n case?
> 
> Yes, I try to optimize for the case.

I'd suggest either optimizing in _common_ code, or not optimizing it at
all.  Once in production, and all the config options are on, the
optimization goes away anyway.

You could create a hpagecache_nr_pages() helper or something I guess.

>>> +	}
>>> +	__mod_zone_page_state(page_zone(page), NR_FILE_PAGES, nr);
>>> +	if (PageTransHuge(page))
>>> +		__inc_zone_page_state(page, NR_FILE_TRANSPARENT_HUGEPAGES);
>>> +	mapping->nrpages += nr;
>>> +	spin_unlock_irq(&mapping->tree_lock);
>>> +	radix_tree_preload_end();
>>> +	trace_mm_filemap_add_to_page_cache(page);
>>> +	return 0;
>>> +err:
>>> +	if (i != 0)
>>> +		error = -ENOSPC; /* no space for a huge page */
>>> +	page_cache_release(page + i);
>>> +	page[i].mapping = NULL;
>>
>> I guess it's a slight behaviour change (I think it's harmless) but if
>> you delay doing the page_cache_get() and page[i].mapping= until after
>> the radix tree insertion, you can avoid these two lines.
> 
> Hm. I don't think it's safe. The spinlock protects radix-tree against
> modification, but find_get_page() can see it just after
> radix_tree_insert().

Except that the mapping->tree_lock is still held.  I don't think
find_get_page() can find it in the radix tree without taking the lock.

> The page is locked and IIUC never uptodate at this point, so nobody will
> be able to do much with it, but leave it without valid ->mapping is a bad
> idea.

->mapping changes are protected by lock_page().  You can't keep
->mapping stable without holding it.  If you unlock_page(), you have to
recheck ->mapping after you reacquire the lock.

In other words, I think the code is fine.

>> I'm also trying to figure out how and when you'd actually have to unroll
>> a partial-huge-page worth of radix_tree_insert().  In the small-page
>> case, you can collide with another guy inserting in to the page cache.
>> But, can that happen in the _middle_ of a THP?
> 
> E.g. if you enable THP after some uptime, the mapping can contain small pages
> already.
> Or if a process map the file with bad alignement (MAP_FIXED) and touch the
> area, it will get small pages.

Could you put a comment in explaining this case a bit?  It's a bit subtle.

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