RE: hugepage: Strict page reservation for hugepage inodes

[Zhang, Yanmin]

>>-----Original Message-----
>>From: David Gibson [mailto:david@xxxxxxxxxxxxxxxxxxxxx]
>>Sent: 2006年2月28日 17:15
>>To: Zhang, Yanmin
>>Cc: Andrew Morton; William Lee Irwin; linux-kernel@xxxxxxxxxxxxxxx
>>Subject: Re: hugepage: Strict page reservation for hugepage inodes
>>
>>On Tue, Feb 28, 2006 at 04:53:49PM +0800, Zhang, Yanmin wrote:
>>> [YM] Consider this scenario of multi-thread:
>>> One process has 2 threads. The process mmaps a hugetlb area with 1
>>> huge page and there is a free huge page. Later on, the 2 threads
>>> fault on the huge page at the same time.  The second thread would
>>> fail, and WARN_ON check is triggered, then the second thread is
>>> killed by function hugetlb_no_page.
>>
>>That's why this patch *must* go after my other patch which serializes
>>the allocation->instantiation path.
[YM] Sorry, I didn't see other patches. 


>>> >>+int hugetlb_extend_reservation(struct hugetlbfs_inode_info *info,
>>> >>+			       unsigned long atleast)
>>> >>+{
>>> >>+	struct inode *inode = &info->vfs_inode;
>>> >>+	struct address_space *mapping = inode->i_mapping;
>>> >>+	unsigned long idx;
>>> >>+	unsigned long change_in_reserve = 0;
>>> >>+	struct page *page;
>>> >>+	int ret = 0;
>>> >>+
>>> >>+	spin_lock(&hugetlb_lock);
>>> >>+	read_lock_irq(&inode->i_mapping->tree_lock);
>>> >>+
>>> >>+	if (info->prereserved_hpages >= atleast)
>>> >>+		goto out;
>>> >>+
>>> >>+	/* prereserved_hpages stores the number of pages already
>>> >>+	 * guaranteed (reserved or instantiated) for this inode.
>>> >>+	 * Count how many extra pages we need to reserve. */
>>> >>+	for (idx = info->prereserved_hpages; idx < atleast; idx++) {
>>> >>+		page = radix_tree_lookup(&mapping->page_tree, idx);
>>> >>+		if (!page)
>>> >>+			/* Pages which are already instantiated don't
>>> >>+			 * need to be reserved */
>>> >>+			change_in_reserve++;
>>> >>+	}
>>> [YM] Why always to go through the page cache? prereserved_hpages and
>>> reserved_huge_pages are protected by hugetlb_lock.
>>
>>Erm.. sorry, I don't see how that helps.  We need to go through the
>>page cache to see which pages have already been instantiated, and so
>>don't need to be reserved.
[YM] The huge pages beyond info->prereserved_hpages are always not allocated.

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