Re: write fault on dax mapping and usage of set_pte_at.

From: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
To: Jan Kara <jack@suse.cz>
Cc: Jan Kara <jack@suse.cz>, Chandan Rajendra <chandan@linux.ibm.com>,
	mpe@ellerman.id.au, Dan Williams <dan.j.williams@intel.com>,
	linux-fsdevel@vger.kernel.org
Subject: Re: write fault on dax mapping and usage of set_pte_at.
Date: Thu, 21 Feb 2019 21:27:15 +0530	[thread overview]
Message-ID: <87va1d878k.fsf@linux.ibm.com> (raw)
In-Reply-To: <20190221151513.GB23071@quack2.suse.cz>

Jan Kara <jack@suse.cz> writes:

> On Thu 21-02-19 19:11:14, Aneesh Kumar K.V wrote:
>> On 2/21/19 5:42 PM, Jan Kara wrote:
>> > Hi Aneesh,
>> > 
>> > On Thu 21-02-19 12:52:39, Aneesh Kumar K.V wrote:
>> > > We found this while testing dax with XFS, but i guess this is true for
>> > > other file systems too. The stack trace looks as
>> > > 
>> > >   [c00000000007610c] set_pte_at+0x3c/0x190
>> > > LR [c000000000378628] insert_pfn+0x208/0x280
>> > > Call Trace:
>> > > [c0000002125df980] [8000000000000104] 0x8000000000000104 (unreliable)
>> > > [c0000002125df9c0] [c000000000378488] insert_pfn+0x68/0x280
>> > > [c0000002125dfa30] [c0000000004a5494] dax_iomap_pte_fault.isra.7+0x734/0xa40
>> > > [c0000002125dfb50] [c000000000627250] __xfs_filemap_fault+0x280/0x2d0
>> > > [c0000002125dfbb0] [c000000000373abc] do_wp_page+0x48c/0xa40
>> > > [c0000002125dfc00] [c000000000379170] __handle_mm_fault+0x8d0/0x1fd0
>> > > [c0000002125dfd00] [c00000000037a9b0] handle_mm_fault+0x140/0x250
>> > > [c0000002125dfd40] [c000000000074bb0] __do_page_fault+0x300/0xd60
>> > > [c0000002125dfe20] [c00000000000acf4] handle_page_fault+0x18
>> > > 
>> > > 
>> > > Now that is WARN_ON in set_pte_at which is
>> > > 
>> > > 	VM_WARN_ON(pte_hw_valid(*ptep) && !pte_protnone(*ptep));
>> > > 
>> > > Multiple architecture optimize set_pte_at based on the assumption that
>> > > we will never use set_pte_at to update a valid pte entry. This helps in
>> > > avoid flushing tlb etc. We should be using ptep_set_access_flags for
>> > > this.
>> > 
>> > Hum, I didn't know about this assumption and neither did lot of other
>> > people reviewing DAX patches. Is this documented somewhere?
>
> Any answer here?
>

Not in the form of kernel documentation. But we had related
fixes which got applied earlier

cee216a696b2004017a5ecb583366093d90b1568
56eecdb912b536a4fa97fb5bfe5a940a54d79be6

Which also imply we possibly want to document this properly.

We use:
For a RO -> RW: update ptep_set_access_flags 
From an invalid pte to valid: set_pte_at
For read/modify/write sequence we have: ptep_prot_modify_start/ptep_prot_modify_commit

ptep_set_access_flags do carry a comment  which explains it is used
when relaxing the access.

-aneesh