linux-kernel - Re: [PATCH] mm, fs: avoid page allocation beyond i

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Message-Id: <20130822151614.2F11DE0090@blue.fi.intel.com>
Date:	Thu, 22 Aug 2013 18:16:14 +0300 (EEST)
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Steven Whitehouse <swhiteho@...hat.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Jan Kara <jack@...e.cz>, Al Viro <viro@...iv.linux.org.uk>,
	NeilBrown <neilb@...e.de>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] mm, fs: avoid page allocation beyond i_size on read

Steven Whitehouse wrote:
> Hi,
> 
> On Thu, 2013-08-22 at 17:30 +0300, Kirill A. Shutemov wrote:
> [snip]
> > > Andrew's proposed solution makes sense to me, and is probably the
> > > easiest way to solve this.
> > 
> > Move check to no_cached_page?
> Yes
> 
> > I don't see how it makes any difference for
> > page cache miss case: we anyway exclude ->readpage() if it's beyond local
> > i_size.
> > And for cache hit local i_size will be most likely cover locally cached
> > pages.
> The difference is that as the function is currently written, you cannot
> get to no_cached_page without first calling page_cache_sync_readahead(),
> i.e. ->readpages() so that i_size will have been updated, even if
> ->readpages() doesn't return any read-ahead pages.
> 
> I guess that it is not very obvious that a call to ->readpages is hidden
> in page_cache_sync_readahead() but that is the path that should in the
> common case provide the pages from the fs, rather than the ->readpage
> call thats further down do_generic_file_read()

I've checked the codepath before and to me it looks like ->readpages()
will not be called beyond i_size. Codepath is:

page_cache_sync_readahead()
 ondemand_readahead()
  __do_page_cache_readahead()
   read_pages()
    mapping->a_ops->readpages()

But if you check __do_page_cache_readahead():

152 static int
153 __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
154                         pgoff_t offset, unsigned long nr_to_read,
155                         unsigned long lookahead_size)
156 {
...
163         loff_t isize = i_size_read(inode);
164
165         if (isize == 0)
166                 goto out;
167
168         end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);
...
173         for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
174                 pgoff_t page_offset = offset + page_idx;
175
176                 if (page_offset > end_index)
177                         break;
...
193         }
...
200         if (ret)
201                 read_pages(mapping, filp, &page_pool, ret);
202         BUG_ON(!list_empty(&page_pool));
203 out:
204         return ret;
205 }

Do I miss something?

-- 
 Kirill A. Shutemov
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