Re: smp race fix between invalidate_inode_pages* and do_no_page

[Nick Piggin <nickpiggin@yahoo.com.au>]

Andrea Arcangeli wrote:
> On Wed, Jan 11, 2006 at 03:08:31PM +1100, Nick Piggin wrote:
> 
>>I'd be inclined to think a lock_page is not a big SMP scalability
>>problem because the struct page's cacheline(s) will be written to
>>several times in the process of refcounting anyway. Such a workload
>>would also be running into tree_lock as well.
> 
> 
> I seem to recall you wanted to make the tree_lock a readonly lock for
> readers for the exact same scalability reason? do_no_page is quite a

I think Bill Irwin or Peter Chubb made the tree_lock a reader-writer
lock back in the day.

I have some patches (ref:lockless pagecache) that completely removes
the tree_lock from read-side operations like find_get_page and
find_lock_page, and turns the write side back into a regular spinlock.
You must be thinking of that?

> fast path for the tree lock too. But I totally agree the unavoidable is
> the atomic_inc though, good point, so it worth more to remove the
> tree_lock than to remove the page lock, the tree_lock can be avoided the
> atomic_inc on page->_count not.
> 

Yep, my thinking as well.

> The other bonus that makes this attractive is that then we can drop the
> *whole* vm_truncate_count mess... vm_truncate_count and
> inode->trunate_count exists for the only single reason that do_no_page
> must not map into the pte a page that is under truncation. We can
> provide the same guarantee with the page lock doing like
> invalidate_inode_pages2_range (that is to check page_mapping under the
> page_lock and executing unmap_mapping_range with the page lock held if
> needed). That will free 4 bytes per vma (without even counting the
> truncate_count on every inode out there! that could be an even larger
> gain), on my system I have 9191 vmas in use, that's 36K saved of ram in
> my system, and that's 36K saved on x86, on x86-64 it's 72K saved of
> physical ram since it's an unsigned long after a pointer, and vma must
> not be hw aligned (and infact it isn't so the saving is real). On the
> indoes side it saves 4 bytes
> * 1384 on my current system, on a busy nfs server it can save a lot
> more. The inode also most not be hw aligned and correctly it isn't. On a
> server with lot more of vmas and lot more of inodes it'll save more ram.
> 
> So if I make this change this could give me a grant for lifetime
> guarantee of seccomp in the kernel that takes less than 1kbyte on a x86,
> right? (on a normal desktop I'll save at minimum 30 times more than what
> I cost to the kernel users ;) Just kidding of course...
> 

Sounds like a good idea (and your proposed implementation -
lock_page and recheck mapping in do_no_page sounds sane).

Thanks,
Nick

-- 
SUSE Labs, Novell Inc.

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