linux-ext4 - Re: [PATCHv3 14/41] filemap: allocate huge page in page_cache

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Date:   Tue, 11 Oct 2016 18:15:45 +0200
From:   Jan Kara <jack@...e.cz>
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>,
        Matthew Wilcox <willy@...radead.org>,
        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: [PATCHv3 14/41] filemap: allocate huge page in
 page_cache_read(), if allowed

On Thu 15-09-16 14:54:56, Kirill A. Shutemov wrote:
> This patch adds basic functionality to put huge page into page cache.
> 
> At the moment we only put huge pages into radix-tree if the range covered
> by the huge page is empty.
> 
> We ignore shadow entires for now, just remove them from the tree before
> inserting huge page.
> 
> Later we can add logic to accumulate information from shadow entires to
> return to caller (average eviction time?).
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  include/linux/fs.h      |   5 ++
>  include/linux/pagemap.h |  21 ++++++-
>  mm/filemap.c            | 148 +++++++++++++++++++++++++++++++++++++++++++-----
>  3 files changed, 157 insertions(+), 17 deletions(-)
> 
...
> @@ -663,16 +663,55 @@ static int __add_to_page_cache_locked(struct page *page,
>  	page->index = offset;
>  
>  	spin_lock_irq(&mapping->tree_lock);
> -	error = page_cache_tree_insert(mapping, page, shadowp);
> +	if (PageTransHuge(page)) {
> +		struct radix_tree_iter iter;
> +		void **slot;
> +		void *p;
> +
> +		error = 0;
> +
> +		/* Wipe shadow entires */
> +		radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, offset) {
> +			if (iter.index >= offset + HPAGE_PMD_NR)
> +				break;
> +
> +			p = radix_tree_deref_slot_protected(slot,
> +					&mapping->tree_lock);
> +			if (!p)
> +				continue;
> +
> +			if (!radix_tree_exception(p)) {
> +				error = -EEXIST;
> +				break;
> +			}
> +
> +			mapping->nrexceptional--;
> +			rcu_assign_pointer(*slot, NULL);

I think you also need something like workingset_node_shadows_dec(node)
here. It would be even better if you used something like
clear_exceptional_entry() to have the logic in one place (you obviously
need to factor out only part of clear_exceptional_entry() first).

> +		}
> +
> +		if (!error)
> +			error = __radix_tree_insert(&mapping->page_tree, offset,
> +					compound_order(page), page);
> +
> +		if (!error) {
> +			count_vm_event(THP_FILE_ALLOC);
> +			mapping->nrpages += HPAGE_PMD_NR;
> +			*shadowp = NULL;
> +			__inc_node_page_state(page, NR_FILE_THPS);
> +		}
> +	} else {
> +		error = page_cache_tree_insert(mapping, page, shadowp);
> +	}

And I'd prefer to have this logic moved to page_cache_tree_insert() because
logically it IMHO belongs there - it is a simply another case of handling
of radix tree used for page cache.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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