[Devel] Re: [RFC][PATCH] EXT3: problem with page fault inside a transaction

[Nick Piggin]

Dmitriy Monakhov wrote:
> Andrew Morton <akpm at osdl.org> writes:

>>With the stuff Nick and I are looking at, we won't take pagefaults inside
>>prepare_write()/commit_write() any more.
> 
> I'sorry may be i've missed something, but how cant you prevent this?
> 
> Let's look at generic_file_buffered_write:
> #### force page fault
> fault_in_pages_readable();
> 
> ### find and lock page
>  __grab_cache_page()
> 
> #### allocate blocks.This may result in low memory condition
> #### try_to_free_pages->shrink_caches() and etc.  
> a_ops->prepare_write() 		
> 
> ### can anyone guarantee that page fault hasn't  happened by now ?

Yes. Do an atomic copy, which will early exit from the pagefault handler
and return a short copy. Then close up the write, drop the page lock,
and rerun the fault_in_pages_readable, which will do the full pagefaults
for us, then try again.

Regardless of what you do to ext3, the VM just can't handle a fault
here anyway.

-- 
SUSE Labs, Novell Inc.
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