linux-kernel - Re: [PATCH v7 07/22] Replace the XIP page fault handler with the DAX page fault handler

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Message-ID: <20140729210457.GA17807@quack.suse.cz>
Date:	Tue, 29 Jul 2014 23:04:57 +0200
From:	Jan Kara <jack@...e.cz>
To:	Matthew Wilcox <willy@...ux.intel.com>
Cc:	Jan Kara <jack@...e.cz>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 07/22] Replace the XIP page fault handler with the DAX
 page fault handler

On Tue 29-07-14 08:12:59, Matthew Wilcox wrote:
> On Wed, Apr 09, 2014 at 11:43:31PM +0200, Jan Kara wrote:
> > So there are three places that can fail after we allocate the block:
> > 1) We race with truncate reducing i_size
> > 2) dax_get_pfn() fails
> > 3) vm_insert_mixed() fails
> > 
> > I would guess that 2) can fail only if the HW has problems and leaking
> > block in that case could be acceptable (please correct me if I'm wrong).
> > 3) shouldn't fail because of ENOMEM because fault has already allocated all
> > the page tables and EBUSY should be handled as well. So the only failure we
> > have to care about is 1). And we could move ->get_block() call under
> > i_mmap_mutex after the i_size check.  Lock ordering should be fine because
> > i_mmap_mutex ranks above page lock under which we do block mapping in
> > standard ->page_mkwrite callbacks. The only (big) drawback is that
> > i_mmap_mutex will now be held for much longer time and thus the contention
> > would be much higher. But hopefully once we resolve our problems with
> > mmap_sem and introduce mapping range lock we could scale reasonably.
> 
> Lockdep barfs on holding i_mmap_mutex while calling ext4's ->get_block.
> 
> Path 1:
> 
> ext4_fallocate ->
>  ext4_punch_hole ->
>   ext4_inode_attach_jinode() -> ... ->
>     lock_map_acquire(&handle->h_lockdep_map);
>   truncate_pagecache_range() ->
>    unmap_mapping_range() ->
>     mutex_lock(&mapping->i_mmap_mutex);
  This is strange. I don't see how ext4_inode_attach_jinode() can ever lead
to lock_map_acquire(&handle->h_lockdep_map). Can you post a full trace for
this?

> Path 2:
> do_dax_fault() ->
>  mutex_lock(&mapping->i_mmap_mutex);
>  ext4_get_block() -> ... ->
>   lock_map_acquire(&handle->h_lockdep_map);
  This is obviously correct.

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