Re: Ext4 stack trace with savedwrite patches

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Wednesday 01 March 2017 03:19 PM, Jan Kara wrote:
> Hi,
>
> On Fri 24-02-17 19:23:52, Aneesh Kumar K.V wrote:
>> I am hitting this while running stress test with the saved write patch
>> series. I guess we are missing a set page dirty some where. I will
>> continue to debug this, but if you have any suggestion let me know.
> <snip>
>
> So this warning can happen when page got dirtied but ->page_mkwrite() was
> not called. I don't know details of how autonuma works but a quick look
> suggests that autonuma can also do numa hinting faults for file pages.
> So the following seems to be possible:
>
> Autonuma decides to check for accesses to a mapped shared file page that is
> dirty. pte_present gets cleared, pte_write stays set (due to logic
> introduced in commit b191f9b106 "mm: numa: preserve PTE write permissions
> across a NUMA hinting fault"). Then page writeback happens, page_mkclean()
> is called to write-protect the page. However page_check_address() returns
> NULL for the PTE (__page_check_address() returns NULL for !pte_present
> PTEs) so we don't clear pte_write bit in page_mkclean_one().


Even though we cleared _PAGE_PRESENT a pte_present() check return true 
for numa fault pte. The problem with savedwrite patch series that i 
quoted in the original mail was that pte_write() was checking on 
_PAGE_WRITE where as numa fault stashed the write bit as savedwrite bit. 
Hence page_mkclean was skipping those ptes.

> Sometime later
> a process looks at the page through mmap, takes NUMA fault and
> do_numa_page() reestablishes a writeable mapping of the page although the
> filesystem does not expect there to be one and funny things happen
> afterwards...
>
> I'll defer to more mm-savvy people to decide how this should be fixed. My
> naive understanding is that page_mkclean_one() should clear the pte_write
> bit even for pages that are undergoing NUMA probation but I'm not sure
> about a preferred way to achieve that...
>
>


Yes found that and finally decided that instead of fixing all those code 
path, we can update pte_write to handle autonuma preserved write bit.

https://lkml.kernel.org/r/1488203787-17849-2-git-send-email-aneesh.kumar@linux.vnet.ibm.com

-aneesh

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